The CSS validator uses a different form variable and a different URL. This seems to work (Gmail is going to break this up into multiple lines ... be sure it's all one line in TextMate):

curl -o /tmp/`id -u`/results.html -F file=@$TM_FILEPATH http://jigsaw.w3.org/css-validator/validator && open /tmp/`id -u`/results.html

OK, very nice, it works fine :)