If it's https it's pretty secure.

If it's http then it is possible for a tor node to sniff the data. Anything you send over http would be visible. Which means logging into a site that doesn't use https you'd expose your login/password. TradeHill, MtGox all use https so that's not a problem. Gmail is 100% https now I believe also. All banks are https.

This forum only uses https for your login. Which means people could sniff your cookie while you browse/post.

Anyway, as long as it's https then you're fine. Anything non-https is less secure then your internet connection at home. The odds of someone sniffing one of your exit nodes, is probably pretty slim.