I have to say that if you're not intent on using the N900 for being a truly mobile computer or interested in it's Linux side, I would go for a Droid.

The hardware is very similar, but Android has a slightly more refined 'phone' side, as well as a bigger set of easy to use and grab apps.

If you're in the US you get Google Nav, too.

That being said, the browsing on the N900 is top notch, so if you're getting it for that and don't mind maybe having to do a few hacks now and then, it'll be a fine phone/computer.