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.<p>For example:
<a href="http://www.teamfurry.com/wordpress/2007/11/20/tor-exit-node-doing-mitm-attacks" rel="nofollow">http://www.teamfurry.com/wordpress/2007/11/20/tor-exit-node-...</a><p>A google search returns lots of results, although I'm unsure if any are 'in the wild' exploits.