Use the Crystal IsNull function........

IF ({LeaseMaster.LeaseDispositionDate} >= DateTime (2009, 01, 01, 00, 00, 00)
OR  IsNull{LeaseMaster.LeaseDispositionDate})
THEN "LSE"
ELSE "NOT"

Actually, you should ALWAYS check for IsNull first.  When you compare a null against a value, the result is null instead of True or False.  Crystal, like SQL and most programming languages, will stop evaluating a boolean expression that contains "and" or "or" as soon as it gets a non-False value (which is not the same as a True value!) for a part of the expression.  Since null is a non-False value you always want to look for nulls first.