EDIT: I can't reply to the thread below so I'm putting my reply here. I have activated a half dozen iPhones on my Mac for friends that don't own computers. While it may not be the smartest thing (no backups), it is certainly doable and both the iPad and the iPhone are still very much useful without a computer.
The only unbiased, quantifiable data I can come up with is a simple Google search:
http://www.google.com/search?q=%22iphone+without+computer%22
It appears that there is quite a bit of interest in using the iPhone without a computer based on the resulting articles and forum posts.