Happened to me just the other day
But that was probably because I was running at least 5 or 6 applications at the same time. I was using app windows to make a background that gives you the impression that you are using different applications, but in reality, the windows are just images... I just saved them as screenshots. Looks great...
Anyway, while I was doing this and organizing windows, I found that I couldn't click or drag anything with my wireless Mighty Mouse nor my trackpad. Thing just froze up on me. The whole system was unresponsive, and this is Leopard that I'm using, not Tiger. An old-fashioned, manual shut down did the trick.
Anyway, I'd think that the iMac that we are all talking about is probably fine. Let us know how that gets on.