SQL Server 7 only allows Yes or No in bit field. SQL 2000 introduced the ability to also store Null.
Often you don't want this behaviour.
Note: It causes a bug if your front end if Access
ACC2000: Write Conflict Error When You Try to Update Records in a Linked SQL Server Table (Q280730) http://support.microsoft.com/default.aspx?scid=kb;EN-US;q280730
ACC97: Write Conflict Error When You Try to Update Records in a Linked SQL Server Table (Q278696)
http://support.microsoft.com/default.aspx?scid=kb;EN-US;q278696