If your server/session has ANSI_NULLS ON then any comparison to a NULL value results in Unknown, not true or false. ANSI_NULLS is ON by default in SQL Server so you should use IS NULL/IS NOT NULL when doing comparison to NULL values.
See NULL Comparison Search Conditions in BOL.
Edit: JacekO beat me to it.
Jack Corbett
Consultant - Straight Path Solutions
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