Google and AIM have had interoperability since 2007. You could add your AIM contacts to your Gtalk client and could talk to them within Gmail. However, that restriction has now been removed and you can talk to AIM contacts through a variety of Google services like Orkut, iGoogle and Google Talk on Android phones.
Add your AIM contacts in and chat away
A few differences also now have been made to the existing interoperability. You don't necessarily need to add your AIM account to talk to your AIM contacts, you can talk to them directly from Gtalk by adding them to your list. They too can add your Gtalk account directly to the AIM client they use to chat to your Gtalk. Because of this, you will not be able to sign into your AIM account from Gmail because it's pretty much unnecessary. Finally, if you previously had many AIM contacts in your Gtalk list and don't want to add them back one by one, AIM has created a tool to mass import your contacts.