I'm afraid it doesn't work that way. Everything runs through Yahoo's servers. If you used a piece of software that shows you the IP addresses of who is connected to you (there are many...) it will show you the IP address of the YAHOO server. Person A sends a message that really goes to the yahoo server, the yahoo server forwards the message to you. You see the IP address of the server, not person A. I have no idea how AIM works, it might be different.