Although this doesn't offer chat with native speakers online, it's a good place for learners to chat together at this bbc site:
BBC World Service | Learning English | Communicate
I've only tried getting students to chat on-line once in lesson time - it's quite difficult to moderate and to make students feel that they are getting something useful from the experience that they couldn't just get for themselves on second life or other similar sites.
If you try it, tell us how you get on with it!
Clare
ELTgames.com