I've been griping about IM for quite a while, people expect an instant response from you and sometimes expect you to do whatever it is they want instantly. I always sign in under invisible then make myself available to certain people if I'm not busy. If I'm busy I only make myself available to people I know in real life and a couple of customers that I support via IM. I hate when I think the coast is clear so I go available and then someone else sitting invisible, whom I didn't want to talk to in the first place pops up on me..Grrr!
I've been doing the same thing for years now. People will start asking me questions that really belong on the forum, or try to cozy up to me. (Why do they do this? It's just a forum, I'm not the Pope!)