Accountability with Bitcoin transactions
I am no lawyer, but as far as I know there is no specific precedent for this, or none that I could find anyway. Due to this, if you could reasonably prove that you had no intent in supporting that illegal organisation or group I don't see a strong case for any prosecution. Obviously there is a huge amount of grey area here, so what is "reasonable" would be up to interpretation. Also each country would have its own individual laws, which makes this even harder to give a specific answer.

If you are worried, I would recommend that you do two things. 1) Cover your ass by keeping all emails or any correspondence relating to your BTC transactions in a secure place. 2) You can also use tools like PGP, Hushmail, Cryptocat, Zfone, TrueCrypt and Tor to help protect your online anonymity.