For those looking for something with a bit more oomph and better connections etc, check out something like the BeagleBoard or the PandaBoard.
Couple a BeagleBoard with the latest distribution of Angstrom and you have a nice capable little beastie.
The main issue for me is the integration of PowerVR drivers needed to access the OpenGL ES acceleration of the OMAP3 and 4 series SoCs, at this moment I can get some sample apps released by PowerVR and Texas Instruments (the manufacturer of the OMAP line of ARM based SOCs) to run well on my BeagleBoard, but I can't for the life of me write my own little sample apps. Let alone get access to the TI DSP that's also onboard.
I would also like to point out to those saying get Ubuntu onboard, at this moment, the curent release of Ubuntu has serious issues accessing the Cortex A8 chips at anything about 700Mhz, i'm hoping it'll be fixed soon, but my Angstrom distro has no issues running my BeagleBoard at 1Ghz.
Of course, there is also the Ardruino line of products too.
That said, the R-Pi is a very nice bit of packaging, it's just a shame they have used such an underpowered ARM chip, from what I can tell it doesn't even have a hardware FP unit and it certainly doesn't have the ARM NEON extensions (i.e. ARMs version of MMX/SSE/3D NOW! etc)