Um, is there a question in there somewhere? You didn't say what preferences you turned off, or why.

Does you friend have more than one computer? Or maybe one at his office was left on? The AIM message almost certainly means that your friend was logged in to the same account on multiple computers. But the message also gives you the opportunity to log off on the other computer, not currently in use.

I am using 10.5.2 and that problem just started for me also. As soon as I open iChat, it logs on and off AIM repeatedly as fast as it can, and then AOL blocks the account for too many tries. Yahoo messenger doesn't have any problems. I tried using Adium, and that works after you wait 10 minutes or soon

Repairing permissions doesn't do anything
but logging off and trying the same AIM account using a different OS X user worked.

The only (kind-of) solution I can find so far was to run DiskWarrior on my startup drive, and rebuild. After that, it only made one quick log in, log off, before logging in and staying put. I have the feeling that this will be a temporary fix, and I will have to repeat the process soon though.

Anyone have an idea if it's Leopard or iChat, or AOL that is the problem?

What I did to fix the problem was delete the user from my ichat accounts, by pressing the minus sign. Then waiting a couple minutes just to make sure that I would not get the logged in too many times error and re added the account. And as for now it is working fine and all is normal.