I've never heard of this thing before, but the Ben NanoNote from Qi Hardware looks like it could possibly hide out in the tiny coin pocket on a pair of Levi's, let alone be a fully-hackable Linux handheld device. Available now for only $99, the NanoNote is pretty basic with a 3-inch 320×240 display and a 336MHz XBurst Jz4720 processor. No, that's not a misprint; we're talking about a MIPS (not ARM-based) CPU, along with 32MB RAM, 2GB Flash storage, and the obligatory microSD slot.
While I'm struggling to think of a use for this, I'm sure there are plenty of our readers out there who would be able to mold this device into a variety of things, like say an electronic dictionary or basic eBook reader. In fact, someone has actually already created a 3D game on the NanoNote. With that XBurst processor, I'm sure the battery life is phenomenal, and the price puts it in that "impulse buy" category.
Check out the NanoNote's product page for a variety of links and if the UMID BZ and Fujitsu UH900 are a bit too large for you, then at least now there is an alternative, as long as no-frills computing suits you.