Two weeks ago I bought a PX10000 the first available pico-itx board for one of my projects. The thing is really really small 10cm x 7.2cm (smaller then the Nokia N800 as you can see on the picture). The board seems too be 100% Linux compatible (I haven't tried the SATA and PATA controllers yet) once you find that you need the openChrome Experimental Branch to get your X running nicely (VESA driver works but sucks as all ways). The thing really boots from everything you connect to it: hard disk, cdrom, usb-stick, usb-disk, usb-cdrom (I installed ubuntu 7.04 from a usb-cdrom to a usb-disk).
I only had a little trouble with the Wake-on-Lan functionality, since there is no setting in the BIOS and it didn't just work. The trick is to enable wake on PCI card event, after that use ethtool to set the wakeup type and you're done.
The board seems quite fast (I gave it 1GB of memory). The only annoying thing with this board is the fan in the middle of the heat sink. Also the fan is not to noisy I really would like to see a version that is cooled passively.
The bad part is that PX10000 is quite expensive compared to a Mini-ITX board, the PX10000 comes around 230 Euro while you get a Mini-ITX for around 130 Euro.
If you really need something small for a project I think this is a good choice.
A more complete review can be found at: Mini-itx.com