Good question, and im not 100% sure of the reason why. However, if you use the following code the select statement works fine

declare @dec varchar(50)

set @dec = ( select DecryptByCert(Cert_id('DBCert'), [Status])

FROM dbo.Status)

select @dec

so the whole code is

create MASTER KEY

ENCRYPTION BY PASSWORD = 'Pass123'

GO

/*

create ASYMMETRIC KEY Asym_Key

WITH ALGORITHM = RSA_512

ENCRYPTION BY PASSWORD = 'Pass123'

GO

create SYMMETRIC KEY Sym_Key

WITH ALGORITHM = DES

ENCRYPTION BY PASSWORD = 'Pass123'

GO

*/

--select * from sys.certificates

create CERTIFICATE DBCert

WITH SUBJECT = 'Self-Certification',

START_DATE = '26-June-2008',

EXPIRY_DATE = '28-June-2008'

GO

create TABLE [dbo].[Status]

(

[StatusID] [tinyint] IDENTITY(1,1) NOT NULL,

[Status] varchar(500) NOT NULL,

CONSTRAINT [PK_CourseStatus] PRIMARY KEY CLUSTERED

(

[StatusID] ASC

)

) ON [PRIMARY]

SELECT * FROM dbo.Status

GO

INSERT dbo.Status( Status )

VALUES ( EncryptByCert( Cert_id('DBCert'), 'EncryptedStatus') )

SELECT * FROM dbo.Status

GO

declare @dec varchar(50)

set @dec = ( select DecryptByCert(Cert_id('DBCert'), [Status])

FROM dbo.Status)

select @dec

--drop ASYMMETRIC KEY Asym_Key

--drop SYMMETRIC KEY Sym_Key

drop CERTIFICATE DBCert

drop TABLE [dbo].[Status]

drop MASTER KEY

Hope this can help you get round the problem, however i am still intruiged as to how the origional select doesnt work?!