How many users actually use all the functionality?
My sister bought a 15" MacBook Pro for Xmas. She checks work e-mail via Outlook Web Access, occasionally works with MS Office documents for the office, uses an IM client, YouTube, browses the web for vacation ideas/planning, iTunes, iPhoto and... that's it. Not a luddite. Quite savvy and bright. Only 30. Yet that's all she does with a $1700 laptop.
Granted, you could do the same with a netbook, however, the iPad has nothing to really "mess up". There's not much to configure. There's not much updating. It's instant on. It has better battery life. For her intents and purposes, it does 95% of what she wants. I can say this for my parents, and either most of my friends, or their spouses.
For these types of users the lack of a full-blown OS is actually an advantage. There's nothing "to mess up". There's very few things to configure. No boot times. Longer battery life. Easier to carry around. Cheaper than a conventional laptop or desktop. Can be always online (WiFi or 3G). Presumably a very simple "restore" or factory default reset process. I would also assume you could get a MobileMe account and have all your data backed up in a data center somewhere for easy restore.
I, personally, am not sold on the device. I already have an iPhone, and I have different computing needs as a developer. However I won't ignore the fact that it'll cut the mustard for a large number of users as an "occasional" computer, if not a primary computing device.