It's interesting that you guys are always talking about having an iMac and an iBook, instead of a Powerbook, as if the base prices are all to consider. So untrue, it's silly, really.
First, you have to pay for AppleCare on two items. Second, you have to buy all the necessary accessories for the laptop, which in my case, ran me an extra 600 bucks (RAM, sleeve, bag, keyboard cover, tiny software apps, upgrade to Tiger (or Leopard in a year, family license)).
Total cost of ownership of my 12 inch Powerbook w/Applecare and edu discount to date? Around 2000. Now throw an iMac into the mix...
Buy a powerful laptop and the iMac becomes completely unnecessary. Wait for the intel Powerbooks and watch that puppy fly. Unless there is a huge, compelling, dead computer at home, you can wait a year and see what happens. Then, buy a pimped out Powerbook and enjoy it for 4-5 years. Or sell it after 2-3 years and still get a lot of money back.
Buying two computers seems like a good idea, but once you buy all the pieces that fit the puzzle, you'll find that total cost of ownership is huge; much, much larger than simply buying one powerbook.