Chris, it appears that SQL Server requires the WITH SCHEMABINDING function option in order for a function to be identified as deterministic.

Here is a completely meaningless sample to show how SQL server handles deterministic functions.

ALTER FUNCTION dbo.fnDateAdd(@intA int)

RETURNS int

WITH SCHEMABINDING

AS

BEGIN

RETURN @intA

END

GO

-- This should return 1

SELECT objectpropertyex(object_id('dbo.fnDateAdd'), N'IsDeterministic')

-- This will only call the function once

SELECT * FROM sysobjects WHERE [id] <= dbo.fnDateAdd(object_id('dbo.fnDateAdd'))

-- This will call the function for every row in sysobjects.

SELECT * FROM sysobjects WHERE [id] <= dbo.fnDateAdd([id])