Nowadays, the next best thing is the Pyra, but that still has't been released.
By the way, have you seen the neo900 project? It sounds like you would be interested, although it is also incomplete.
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. :(
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).