http://discussions.apple.com/thread.jspa?t...2&stqc=true

Copypasta'd from that link:

http://www.hara.ipwn.me/blog/?p=132

This solution worked in 10.5.6. Now that I am on 10.5.7, the issue has resurfaced, and now that solution with 100% worked before no longer works.

Add buddy, buddy appears for a few seconds, then dissapears (doesn't show as offline either). Requires quitting and starting iChat again for the buddy to show either online or in the offline group. This is quite annoying (again), wondering if anyone else has this issue now, and if anyone solved it.

Deleted preferences, deleted address book files, rebooted, no joy. As I said, it worked perfectly on 10.5.6, no longer now that I have 10.5.7 installed.