I setup @BitList https://bit-list.appspot.com
with this in mind.
The login uses your twitter account, so that can be an anon as you want it, in addition you disguise your twitter account with a nickname of your choosing.
It lets you create Message Drops on the map. The idea is you enter some hashtags in the description, such as #bitcoin, #bitcoin-trader, #bitcoin-seller or #bitcoin-buyer.
people can find message drops around them, filtering by any tag. so someone would Search for 'bitcoin' and only message drops with #bitcoin in the description would show on the map. With the messages, you can work out an inperson exchange.
Opening a Drop anyone can add a public message to it as well as a private message to it's owner. I need some way to add more virality to it, but it is a little difficult with it's focus on anonymity.
There is also the ubitex.org project.