MySQL Conditional Trigger

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I don't think my trigger likes a comparison with a variable which is
NULL.  The docs seem to have a few interesting variations on playing
with NULL and I was hoping someone would just throw me a fish so I
don't have to try every permutation (possibly using CASE, IFNULL,
etc.).

If my ShipDate (which is a date datatype which can be NULL) changes to
a non-null value, I want my IF statement to evaluate to TRUE.
IF NULL changes to aDate : TRUE
IF aDate changes to aDifferentDate : TRUE
IF anyDate changes to NULL : FALSE

In my trigger I have:
...
IF OLD.ShipDate != NEW.ShipDate AND NEW.ShipDate IS NOT NULL THEN
...

Which only works when ShipDate was not NULL to begin with.
I suppose it evaluates the following to FALSE
IF NULL != '2008-10-31' AND '2008-10-31' IS NOT NULL THEN
(not liking the "NULL != '2008-10-31'" part)

Please give me the correct syntax.
TIA

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