There's a workaround on this one... but it comes with a cost :

CREATE VIEW dbo.vwTblNameForceUnique

WITH SCHEMABINDING

AS

SELECT PrimaryKey, UniqueCol

FROM dbo.TblName

WHERE UniqueCol IS NOT NULL

GO

--Create clustered index for the view

GO

CREATE UNIQUE INDEX IX_vwTblNameForceUnique_UniqueCol ON dbo.vwTblNameForceUnique (UniqueCol)

GO