I'm sad to find out that even though the Applescript support in Adium 1.2 is rewritten you still can't get the actual message-text from a chat. Isn't that kind of basic? And it was actually implemented back in Adium v0.89.
This is waiting on a project Colin was hacking on a while back. I should ask him about it.
Basically, we'd rather not take the memory hit of storing every message twice (once in the message view, and once internally for applescript), so he's trying to make a way to pull that information from the logs if requested.