Scuttlebutt I've heard is that the really determined ones are copying the image, tossing it up on another site and having a hyumon read it (who then gets free pr0n or what not), and sending the result back. This scheme would be just as vulnerable to something similar.

As a somewhat related aside, I was going to submit something similar to the Obfuscated Perl contest a few years back (but didn't because I used GD and the rules excluded using non-core modules.