Schneier et al Report PGP Vulnerability
SpaceTaxi writes: "Researchers reported that they were able to intercept and modify a PGP encrypted message so that, IF it is sent back to the attacker, the original message could be read by the attacker." The paper comes from Kahil Jallad, Jonathan Katz, and Bruce Schneier. Here is the Yahoo! article.
A larger concern would be if you could send a garbage message to someone, and their vacation auto-reply would send it back to you (after decrypting it).
That's about the stupidest thing I've ever heard, and I'm now dumber for having read it.