To clarify what I predict will be a misperception: This new Gmail feature doesn’t let you talk to AIM contacts from GTalk.
What it does is allow you to sign into AIM from Gmail. In other words, Gmail is now an AIM client as well as a Google Talk client. But you can’t send messages to your AIM-using friends using only a GTalk account: You need an AIM account, and you need to log in with it (in Gmail or otherwise), and you can only send messages to your AIM contacts from your AIM account, not your GTalk account.
So it really doesn’t work much differently from what you already get in Adium. You add both accounts, and Adium (or Gmail) figures out which account to use on the contact you opened a chat with.
My point is that the only people this affects are those who send IMs from Gmail. For the rest of us, this doesn’t really change anything: there’s still no inter-service messaging between AIM and GTalk.