Since AOL linked ICQ and AIM, you can add ICQ users to your iChat buddy list. Add your buddy's ICQ number, not alias, to the AIM field in your Address book. Then open iChat AV and choose to add them. Your Buddy must be using the newest ICQ Lite that supports this and its recommended to use an AIM screen name* rather than your .Mac account alias. AIM screen names can be applied for free at the AIM site.

Now you can see them go on and offline, and vice versa for them. In iChat AV, it's completely seamless. This has been tested so far using AIM screen names, with unpredictable results when using .Mac aliases.

*For Mac users who do not have a .Mac account, applying for an AIM screen name is a workaround that will allow you to use iChat AV with other .Mac iChat AV users and AIM iChat AV users. iChat AV also works seamlessly with AOL Instant Messenger users.