How have you implemented the 'adult filter'?

A good question, but I'm not sure I want to give out details on the specifics right now - sorry.

I may be discussing the site at PyCon, at which point I'll happily let you know what works and what fails.

Thanks..But i think some other one has already on the front page..So safe to add NSFW :)

Sure - done.

Allow people to flag submissions if you want a working 'adult filter'

Ack. Will do.