If you can tolerate the smaller screen size, look for a BT-enabled PDA. I picked up five HP PDA's with WiFi and BT for $20 (total) including the larger batteries, charging cradles, etc. Of course, much less available horsepower but, IMO, a UI shouldn't require lots of MIPS.
(For hand-held interfaces, I find the size of a PDA much more appropriate than that of even the smallest of tablets)
The units I most recently acquired use something MS-related (CE? WM?). In the past, I've installed NetBSD on PDA's so I could develop in an environment to which I am more accustomed.
However, I now "talk RDP" in my application ("controlled device") so I can just use the PDA as a TS client and *not* have to develop any code to run on the PDA.
Keep in mind, I am using these things for one-offs -- usually rapid prototyping deployments so I can show someone what a device would look like without having to design/implement final hardware/software.
And, the UI is an "optional accessory" in each case. I.e., it "bolts onto" (figuratively) the device; the device doesn't require the UI in order to perform its normal function (at least, those parts that do not involve "talking to the user").
I keep an eye open for cruft like this -- ebay, auctions, CL, etc. Not the sort of things you can *plan* on picking up whenever you want/need them...
I've accumulated a fair selection over the years ranging in size, capabilities, etc. E.g., I have some very small cell phones that are also WiFi/BT enabled so I can keep the UI in my pocket instead of having to carry something large (like a full size PDA or a tablet). Some even have "sliding keyboards", etc. So, you can have a "capable" interface in a really tiny package. (you might consider older generation phones, as well -- if you can tolerate the smaller screen)
[I am keen on reducing the physical size of UI's so they are less significant in a product's design]