The "biggest deal" visitor (IMO) I saw on AT was probably that guy from Runic Games who wrote (or recorded I don't remember) the music for Diablo and Torchlight. I think he only posted in that one thread though.
Oh, and I think I saw a thread in P&N where people were talking about a politician, and either the politician or his PR rep came on and responded. Again, only for that post though.
Anyone know of any others, past or present that are known to be a big deal? I'm curious if we are among the famous here at AT