Another interesting project which might fill my desire for a true mobile computer (if only I could afford it) is the Pyra, the more powerful successor to the Pandora. But at $~500-$~600, it may not be worth it, and it still hasn't actually released yet (although the company did make the Pandora, so they have a history of Getting Stuff Done).
If you're going on the cheap, you can always hack together a Raspberry Pi and a tiny screen, keyboard, and battery into something semi-workable, and use a phone for the rest.
You can get a GSM board from Adafruit fairly cheaply, which would give you 3G data and voice/SMS on a Pi. You'd have to do your own wiring, but if you're building something like this that wouldn't be a problem. Throw a Ting SIM in there (or your other favorite carrier) and you're good to go.
Basically a mobile phone version of Bunnie's Novena laptop, though not quite as open source (at least not until Broadcom finishes opening up the RPi's processor).
The n900 is everything that I thought the g1 was going to be, but wasn't. In a way, it was my fault for blindly creating my own hype over a first generation product I knew nothing about. The g1 turned out to be my bad and disappointing foray into smart phones.
Also, yes I have heard of the neo900 project, but am sad that the project is so far from its goals. There will never be enough n900 users to meet a crowdfund. :(