As a follow up to the earlier Add ICQ users to iChat AV hint...
You are now able to log into iChat using your ICQ account. I discovered this today, after doing the 2004-09-16 security update. So now, when you insert your account details, you are able to just put in your ICQ number, as seen here. iChat should now connect to the ICQ network and retrieve all your ICQ contacts. However, they will all have numbers as their usernames, so you will need to manually change these yourselves.
Also, I have discovered that when communicating with non-iChat ICQ clients, they will have problems with formatting. View these screenshots to see what I'm talking about. I have sent feedback to the iChat team about this; hopefully we'll see a fix in the future.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2004091700084674