If you maintain two long sessions within the same Tor Browser Bundle instance, you're screwed -- not because the exit nodes might be watching you, but because the web sites' logs can be correlated, and the *sequence* of exit nodes that your Tor client chose is very likely to be unique.
Ah, okay, I did not know that. Thanks for that information. I was just wondering how the proposed changes to the private browsing mode would avoid being tracked by exit mixes (as the blog post claimed).
Georg