Another way might be (untested):

WHERE ISNULL(mydatefield, '19000101') = ISNULL(@dateparm, '19000101')

assuming Jan 1st, 1900 is never actually used in your application. Personnaly, I prefer the other method, since I find it more readable and obvious.

>How about:
>
>

WHERE (@dateparm IS NOT NULL AND mydatefield = @dateparm)
>   OR (@dateparm IS NULL     AND mydatefield IS NULL)