AIM is allowing simoutaneous logons. I was able to do it with four different clients at the same time... (on two different machines --second version number of each is the second machine) iChat 1.0.1, 2.0, and fire 0.32g, 0.32h... and it was reproduced on a windows machine using the official AIM client and the latest version of Trillian. Seems to be a AIM server problem.
Edit: Yes, this is a security breach, as someone with your password could sign on at the same time and monitor what others are saying to you (the message from the other person appeared to all four clients), and could message them back as you without you knowing (your sent message only shows on the client that sent it and to the recipient).
Edit: Yes, this is a security breach, as someone with your password could sign on at the same time and monitor what others are saying to you (the message from the other person appeared to all four clients), and could message them back as you without you knowing (your sent message only shows on the client that sent it and to the recipient).