There's no way to have more than one account logged in at the moment, and there's no explicit sign out.
If you go to AIM Settings (Home->Settings->AIM), you can have it set an option to automatically sign out when you quit the app. Also, there you can change the screen name/password it logs in with.
Is AIM always on? Just wondering if AIM can replace the SMS that my wife and I use between each other. So instead of paying for txt messages, just use AIM instead? is that possible?