Actually, in the U.S. Craigslist was just cleared of a lawsuit regarding discrimanatory postings for apartments for rent. The court found that Craigslist was not legally responsible for the content of the material posted (although the posters themselves can and do face legal liability).
See:
http://www.msnbc.msn.com/id/23672479/
While there should be a 'best effort' made, it is unreasonable to monitor every single post on a high traffic site. If it is brought to your attention, you need to take it down. A polite email request from MobiV would have probably worked just fine and we wouldn't be talking about it here.
What amazes me is how may times this scenario continues to play itself out where companies foolishly take this sort of heavy handed (and legally unsupportable) approach and have it backfire on them. You'd think there are enough lessons out there. Of course, there are probably some companies approach these issues a bit more intelligently and you just don't hear about it - which is the whole point.
db