Just curious - is there a way, once an entry is submitted, to withdraw it. I understand I can edit and re-upload my entry, but what about taking it out of a contest

Just contact the site and ask for it to be removed.
http://photoshopcontest.com/contact/index.html

and f that doesn't work you ca nalways edit your entry and post a full white image (with lots of luck you might win with it )

Grefix, I was thinking along the same lines, but a white image, if it dosn't get enough votes to win , will hurt one's VPP average.

The solutution I came up with would be to add some (tasteful) nudity and get kicked out of the contest