The current list of variables to chose from with email subjects is:-
$THING$
$BUGID$
$ACTION$ - added or changed
$SHORTDESC$
$PROJECT$
$CATEGORY$
$ORGANIZATION$
$PRIORITY$
$STATUS$
$TRACKINGID$
However, it would be awesome if there was also a "last submitted by" variable.
(We have a tester who's REALLY annoying... being able to apply a mail filter on his name would be awesome).