As much as I hate to say it, I think tablet-sized devices (Palm, Zaurus or CE) with accessory folding keyboards are the best/only bet right now.
Some of the MobilePros may be able to take NetBSD or Linux, but you'll need a CF to wedge it on, and so forth.
Upshot is that, "in theory," Bluetooth (or WUSB, or Zigbee, or who knows what at this rate) keyboards might be cheap someday, making that combination a bit more 'ain't it cool?' and a bit less 'hunched over a deck of playing cards.' But you might as well pick up a used Transmeta ultralight while you wait.