I just saw this MSN TV Linux Cluster over on Engadget. The boxes have a 733mhz Celeron, 128MB RAM, 2 x USB, Ethernet, and a 64MB CF card for storage. That’s twice the RAM of an Xbox and with a node cost of $0.99 it makes a much more sensible and compact cluster. The only limit right now seems to be a 64MB capacity cap for the CF card.
You do need to build a level shifting serial cable to talk to it though. Microsoft included serial pins on the board, which is convenient. I think that a TTL to RS-232 level shifting box is becoming the second most useful device behind the bench power supply. You need to do serial level shifting whether you are talking to an NSLU, iPod, GP2X, or WRT54G. You might as well make the thing USB while you are at it. So, who wants to do the how-to?