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Use a unique filtered index

On SQL Server 2008 or higher you can simply use a unique filtered index

CREATE UNIQUE INDEX IX_TableName_FormID_isDefault
 ON TableName(FormID)
 WHERE isDefault = 1

Where the table is

CREATE TABLE TableName(
 FormID INT NOT NULL,
 isDefault BIT NOT NULL
)

For example if you try to insert many rows with the same FormId and isDefault set to 1 you will have this error:

Cannot insert duplicate key row in object 'dbo.TableName' with unique
index 'IX_TableName_FormID_isDefault'. The duplicate key value is (1).

Source: http://technet.microsoft.com/en-us/library/cc280372.aspx

From a normalization perspective, this would be an inefficient way of storing a single fact.

I would opt to hold this information at a higher level, by storing (in a different table) a foreign key to the identifier of the row which is considered to be the default.

CREATE TABLE [dbo].[Foo](
 [Id] [int] NOT NULL,
 CONSTRAINT [PK_Foo] PRIMARY KEY CLUSTERED 
(
 [Id] ASC
) ON [PRIMARY]
) ON [PRIMARY]

GO

CREATE TABLE [dbo].[DefaultSettings](
 [DefaultFoo] [int] NULL
) ON [PRIMARY]

GO

ALTER TABLE [dbo].[DefaultSettings] WITH CHECK ADD CONSTRAINT [FK_DefaultSettings_Foo] FOREIGN KEY([DefaultFoo])
REFERENCES [dbo].[Foo] ([Id])
GO

ALTER TABLE [dbo].[DefaultSettings] CHECK CONSTRAINT [FK_DefaultSettings_Foo]
GO

You could use an insert/update trigger.

Within the trigger after an insert or update, if the count of rows with isDefault = 1 is more than 1, then rollback the transaction.

CREATE VIEW vOnlyOneDefault
AS
 SELECT 1 as Lock
 FROM <underlying table>
 WHERE Default = 1
GO
CREATE UNIQUE CLUSTERED INDEX IX_vOnlyOneDefault on vOnlyOneDefault (Lock)
GO

You'll need to have the right ANSI settings turned on for this.

I don't know about SQLServer.But if it supports Function-Based Indexes like in Oracle, I hope this can be translated, if not, sorry.

You can do an index like this on suposed that default value is 1234, the column is DEFAULT_COLUMN and ID_COLUMN is the primary key:

CREATE 
UNIQUE 
 INDEX only_one_default 
 ON my_table
 ( DECODE(DEFAULT_COLUMN, 1234, -1, ID_COLUMN) )

This DDL creates an unique index indexing -1 if the value of DEFAULT_COLUMN is 1234 and ID_COLUMN in any other case. Then, if two columns have DEFAULT_COLUMN value, it raises an exception.

The question implies to me that you have a primary table that has some child records and one of those child records will be the default record. Using address and a separate default table here is an example of how to make that happen using third normal form. Of course I don't know if it's valuable to answer something that is so old but it struck my fancy.

--drop table dev.defaultAddress;
--drop table dev.addresses;
--drop table dev.people;

CREATE TABLE [dev].[people](
 [Id] [int] identity primary key,
 name char(20)
)
GO

CREATE TABLE [dev].[Addresses](
 id int identity primary key,
 peopleId int foreign key references dev.people(id),
 address varchar(100)
) ON [PRIMARY]

GO
CREATE TABLE [dev].[defaultAddress](
 id int identity primary key,
 peopleId int foreign key references dev.people(id),
 addressesId int foreign key references dev.addresses(id))
go
create unique index defaultAddress on dev.defaultAddress (peopleId)
go
create unique index idx_addr_id_person on dev.addresses(peopleid,id);
go
ALTER TABLE dev.defaultAddress
 ADD CONSTRAINT FK_Def_People_Address
 FOREIGN KEY(peopleID, addressesID)
 REFERENCES dev.Addresses(peopleId, id)
go
insert into dev.people (name) 
 select 'Bill' union 
 select 'John' union 
 select 'Harry'
insert into dev.Addresses (peopleid, address) 
 select 1, '123 someplace' union 
 select 1,'work place' union
 select 2,'home address' union
 select 3,'some address'
insert into dev.defaultaddress (peopleId, addressesid)
 select 1,1 union
 select 2,3 
-- so two home addresses are default now
-- try adding another default address to Bill and you get an error
select * from dev.people 
 join dev.addresses on people.id = addresses.peopleid
 left join dev.defaultAddress on defaultAddress.peopleid = people.id and defaultaddress.addressesid = addresses.id 
insert into dev.defaultaddress (peopleId, addressesId)
 select 1,2 

GO

You could do it through an instead of trigger, or if you want it as a constraint create a constraint that references a function that checks for a row that has the default set to 1

EDIT oops, needs to be <=

Create table mytable(id1 int, defaultX bit not null default(0))
go

create Function dbo.fx_DefaultExists()
returns int as 
Begin
 Declare @Ret int
 Set @ret = 0
 Select @ret = count(1) from mytable 
 Where defaultX = 1

 Return @ret
End
GO
Alter table mytable add
CONSTRAINT [CHK_DEFAULT_SET] CHECK 
 (([dbo].fx_DefaultExists()<=(1)))
GO
Insert into mytable (id1, defaultX) values (1,1)

Insert into mytable (id1, defaultX) values (2,1)

This is a fairly complex process that cannot be handled through a simple constraint.

We do this through a trigger. However before you write the trigger you need to be able to answer several things:

do we want to fail the insert if a default exists, change it to 0 instead of 1 or change the existing default to 0 and leave this one as 1?
what do we want to do if the default record is deleted and other non default records are still there? Do we make one the default, if so how do we determine which one?

You will also need to be very, very careful to make the trigger handle multiple row processing. For instance a client might decide that all of the records of a particular type should be the default. You wouldn't change a million records one at a time, so this trigger needs to be able to handle that. It also needs to handle that without looping or the use of a cursor (you really don't want the type of transaction discussed above to take hours locking up the table the whole time).

You also need a very extensive tesing scenario for this trigger before it goes live. You need to test:
adding a record with no default and it is the first record for that customer
adding a record with a default and it is the first record for that customer
adding a record with no default and it is the not the first record for that customer
adding a record with a default and it is the not the first record for that customer
Updating a record to have the default when no other record has it (assuming you don't require one record to always be set as the deafault)
Updating a record to remove the default
Deleting the record with the deafult
Deleting a record without the default
Performing a mass insert with multiple situations in the data including two records which both have isdefault set to 1 and all of the situations tested when running individual record inserts
Performing a mass update with multiple situations in the data including two records which both have isdefault set to 1 and all of the situations tested when running individual record updates
Performing a mass delete with multiple situations in the data including two records which both have isdefault set to 1 and all of the situations tested when running individual record deletes

@Andy Jones gave an answer above closest to mine, but bearing in mind the Rule of Three, I placed the logic directly in the stored proc that updates this table. This was my simple solution. If I need to update the table from elsewhere, I will move the logic to a trigger. The one default rule applies to each set of records specified by a FormID and a ConfigID:

ALTER proc [dbo].[cpForm_UpdateLinkedReport]
 @reportLinkId int,
 @defaultYN bit,
 @linkName nvarchar(150)
as
if @defaultYN = 1
begin
 declare @formId int, @configId int
 select @formId = FormID, @configId = ConfigID from csReportLink where ReportLinkID = @reportLinkId
 update csReportLink set DefaultYN = 0 where isnull(ConfigID, @configId) = @configId and FormID = @formId
end
update
 csReportLink
 set
 DefaultYN = @defaultYN,
 LinkName = @linkName
where
 ReportLinkID = @reportLinkId