There are now several Linux-based PDAs, with more to come. Pricing varies from low ($200 for the Royal Lin@x) to Pocket-PC range ($550 for the upcoming Linux-based version of the Sharp Zaurus 5500). Linuxdevices.com has a summary page, linked below.
I haven't personally used any of these.
From my perspective, as I can afford something a little pricier than the Royal and since the ergonomics look to be clearly superior, the new Zaurus looks right. There appear to be some battery-life issues, though (see the forums at http://zauruszone.com/). Overall, though, it looks like a growing segment with some interesting comers, which may someday soon include Compaq with a commercial version of their Linux-driven iPaq. Again, though, on the high end.
Also interesting is the Simputer project, from India (see http://www.linuxdevices.com/links/LK6687032328.html or http://www.simputer.org/). They're shooting for a price point of U$200 retail, which they estimate is about as much as a village could afford from common funds. It is designed to be a general-purpose computer in a handheld form factor. Nobody's making one in the west, yet, to my knowledge, but the platform looks great, and I can envision it being licensed and built upon for western markets -- making it the next truly universal platform.
Finally, I have seen solver, spreadsheet and interpreter tools for Palm devices. Even 8MB Palms are inexpensive these days.
