If none of the existing MIDs on the market take your fancy, you could always do what HY Research have done and build your own touchscreen marvel, the Beagle MID. Based on the Beagle Board, the DIY mobile internet device packages a 4.3-inch 480 x 272 touchscreen with a custom interface board and Bluetooth.
Video demo after the cut
The Beagle Board is a compact ARM Cortex A8-based single-board computer, with accelerated audio and video decoding, 2D and 3D rendering support for OpenGL ES 2.0 and both S-Video and HDMI outputs. Passively cooled, it requires a mere 2W at most and has USB, SD/MMC, RS-232 and audio in/out connections, among others. While the Beagle MID runs Linux, the board itself can handle that or Windows CE, depending on your preference.
In the case of the HY Research Beagle MID, the display is the same LCD panel as used on the Sony PSP. Aside from the touchscreen, controls include a 5-way joystick, 3-way slider and a number of buttons. The whole thing measures roughly 6.5 x 3.25 x 1.25 inches, but could be slimmed down to just an inch thick with some reworking.