I would take a look at OK Cupid, if you haven't. The other dating sites like Match or eHarmony seem very "find your future husband or wife" oriented. The other end of the scale, AdultFriendFinder or even the Onion / Nerve personals are very ghetto/spammy and seem to be best suited for ugly people into weird sex stuff, or actual prostitutes.
OK Cupid has worked out very well for me in finding people who just want to go out on a few dates and have fun without the pretense of sizing me up for marriage or whether or not I'll fit into a harness in the dungeon.
If there was an iPhone app for OkCupid it would be nearly perfect. (for actual "dating.")