I had a problem where I was adding names to my iChat buddy list, the name would show up and then it would disappear, could not see the person at all. I do have Chax installed so my Growl notifications would tell that the person was sighing off and on but the iChat Buddy list would give no feedback.
To fix the issue I first went and checked AIM Express to verify that iChat did in fact add that person to my buddy list ( and it did ). Then, I quit iChat, erased all four files in ~/Library/Preferences/:
Restarted iChat and entered my login and …. it is worked just fine now.
Note: One thing that I did notice is that even though I had added my friend correctly ( who was using Adium ), I still could not ping him. He had to add me first then ping me. This is not the case with iChat users.