Sharp have a line of PDA’s called Zaurus. These things are nowhere near the mainstream, but they heave huge developer appeal since they run Linux and Java, and you can develop software “on-board” using vi and various compilers.
That was why I bought one off eBay a few weeks ago. Its turned out to be nowhere near as useful for software dev as I thought it would be. However its turned out to be great for taking notes, writing documents and blogging. The size means I can take it places that a laptop shouldn’t go. Plus I can pull stuff in or out of my CVS server when it’s in the docking station back at the bunker. And seeing a Linux boot sequence when you restart it never ceases to be cool..