He uses a rev C 17" PowerBook, and rarely touches anything else.
He doesn't get new computers all the time because moving everything over isn't something he does for no reason. He has one computer because he keeps everything on that one laptop, along with an external HD. He likes having all his mail and documents on the same laptop. He also believes that the 17" PB is fast enough for all his needs, so why bother getting another laptop. However, since his battery just died, and he's the CEO of Apple, he decided to switch over to a 17" MBP very soon.
Steve also has 23" ACD as an external monitor at home. He also has a Cube, but he hasn't used it in a while.
Thanks for the support, Apple fans.
-Steven Jobs