- Mini-ITX computers. These junior-sized motherboards, frequently used for in-car entertainment systems, are quite powerful, relatively cheap, and have lots of options in terms of ports (firewire, USB, serial, ethernet, wireless, bluetooth, PCI-E, etc. However, using one would require an additional battery-powered power supply and dedicated batteries. The other major downside is that I'd need to carry around a monitor, keyboard, and mouse just to be able to do any kinds of programming in the field. This option probably offers the best performance and flexibility, but at the cost of convenience.
- Mini-laptops. There is a flood of these laptops coming onto the market right now. Priced under $1000 and weighing 3 lbs or less, they seem to be a pretty good choice. The best ones (which have yet to actually hit the market as of this writing) are powered by Intel's new Atom processor (the Diamondville variant being teh most desirable right now). They all have USB, but none have firewire. They all package a VIA C7 or Intel Atom processor, 1-2 Gb of RAM, a screen, keyboard and trackpad, and of course, built-in batteries. The main downside is that it's all locked up in a tidy package that inevitably will be mounted quite high on the robot's body, so some work will be involved to keep all that payload stable.
- Smaller-scale hardware. There are various options such as Pico-ITX scale motherboards and Gumstix: tiny computers on a board. They have similar disadvantages to the mini-ITX board, with the additional performance hits inherent to miniaturization. Their low power draw means that they could probably be run off the chassis batteries.
Overall, since this will be a development platform, I think I'll go with a Mini-laptop, either the Asus 901, the MSI wind, or the ECS G10-IL. The advantages of the all-in-one nature of a laptop are worth the loss of flexibility. Later robots can use the mini/picoITX boards if necessary.
Here's a comparison table.