Keep in mind, once iOS 5 and icloud are released, there is no need for the computer.
The one possible thorn in that is that over-the-air iOS updates may require an Airport Extreme or Time Capsule (the rumor is they will have a small area to cache iOS updates to push to devices. It's actually fascinating, they recognize which iOS device you have, which version of iOS, and automatically download and store system updates to push out to the devices when in wifi range). I'm not sure that's going to be entirely possible if someone isn't using an Apple router/TC (and an updated one at that), we'll see. It might be but just take a little while longer to download and install. We need more details from Apple.
As for purchases of Apps, Music, Books, and I'm sure eventually Videos, iCloud will handle all of that.
Once Apple does actually cut the cord from requiring computers for iPads (and iPhones) I think we'll start to see many people who only own iPads for their computer. (Apple still needs to work on making more printers compatible with the iPad)