If you have the time to watch this video, the author of sslstrip talked briefly
about how he sniffed information of the tor users by acting as a tor exit
node. It is quite scary how ignorant some of the tor users are, and how easily sniffing tor is actually done. Start a Tor Relay, Open Ettercap. 2 simple steps which will guarantee a heck lot of passwords. Not that I tried...
Always remember that Tor is secure for the purpose it was made for. In protecting your anonymity by encrypting all the traffic between the TOR nodes and masking your ip address for the final destination. Nothing else!