Well as an iMac owner and now...ta da...iPad owner I would caution that if you were working with just an iPad I think you would find it a bit tedious being that you would most likely still need a wireless keyboard (if you were planning on the typical work applications, and would not have the mouse to move things along). That being said I was at a NASA workshop and a few of the participants used their iPads with a keyboard and were able to do most everything we needed to do. I LOVE my (ok, now OUR iPad as DW and DD have used it more than I have) and think it is the most incredible device but in reality it is like a palm pilot on serious steroids (IMHO). It is a great source of entertainment and information, but I think that even with the productivity suite titles, you will be better off with the fantastic power book...step up and pay a bit more up front and you won't be sorry.
But I did say it was the coolest invention in the world right?