I just bought one of these:
As my wife said while we were walking out of the Fry's, "It blows my mind that you can buy a whole computer in a little box like that." I don't think I can or want to add anything to that.
I got Ubuntu Lucid Server up and running on without really any trouble at all, even with the configuration I chose, of booting the installer from a thumb drive and installing to an SD card. (I'm trying to keep it as low power as possible; we'll see if the SD card pans out.)
My plan is to write some server code for this thing (in Go!) and hook up some peripherals like a web cam, door opening monitor circuits, etc. My aim is somewhere between home automation and a homebrew security system. Most likely I will just end up spying on the cats. Heh heh...
So far it's a pretty impressive little widget. Hopefully things will pan out and I'll get something spiffy out of it all.