It has been stated by Jeff that flags for obscenity have a small time limit.
This means that 5 or 6 (?) people have to flag a post within a few days for it to be deleted for obscenity. Unfortunately, this means that older posts will stick around with flagging them being pretty much useless, since they don't get the traffic to generate the requisite number of flags before the flags expire.
I don't see any reason to put a time limit on obscenity flags, so that eventually enough users will visit the post, that it will get taken care of.
Now that I have editing powers, I try to edit the obscenities out, but that just starts edit wars with those who get a thrill out of verbal pornography.