[showerst]

I'm not 100% certain of this, but I believe even SSL is vulnerable to certain man-in-the-middle attacks when routed through a single point like tor.

For example: http://www.teamfurry.com/wordpress/2007/11/20/tor-exit-node-...

A google search returns lots of results, although I'm unsure if any are 'in the wild' exploits.