I don't know why Yahoo is being so wonky, but if I am showing up as online for AIM, then I'm probably online for Yahoo. It isn't just wrong on TypePad, but my Yahoo profile also shows that I'm not online when I am really on. And no I don't have that little box checked to hide my status.
As for AIM versus Yahoo. Well, I mainly chat on Yahoo, but I'm hoping that someone will use AIM because iChat is such a pretty program (Apple of course!), but I never get to use it.
Update: Ok after thinking about it a few days I'm going to go with iChat for now. You can always use Trillian if you want to chat with me and you are on Yahoo. Or just email me and let me know and I'll sign on to Yahoo.