If any of you guys are familiar with anonymous Internet systems, this is a really cool read. It basically outlines a possible attack against something like Tor by linking together paths of entry/exit to trace (identify) a user.
Tor was one of the items covered in a presentation on online identities that I did last week here on my campus.
How many nodes would have to be compromised to make this a viable attack vecotr? (yeah, yeah, I know, I should RTFM)
Without going into the detail of the article (I wouldn't want to spoil it for you!): a lot.
But you know how it is. A single proof of insecurity is all you really need.