Mark,

NULLs can be confusing. The only way to effectively filter for them is to use the ISNULL() function. NULL is neither T or F. Asking if NULL = Expression always returns NULL even if the expression is NULL. So, if you need to insure that either NULLs are included or that NULLs are excluded you need to use ISNULL() in the filter. Even though your experience showed that NULL was included, I wouldn't depend on that I would check for NULL explicitily.