This is like the best-documented vaporware ever. I hope they prove me wrong soon.
There are other designs recently launched of the same size and capability like BeagleBone or the Gumstix line.
These are both really, really hard to do. The foundation seems to be approaching this goal fairly well; if it is vaporware they have documented it quite well indeed. Having volunteers like their director and trustee Eben, who is an architect at Broadcomm, and trustee and operations director Pete, who is the managing director of a PCB manufacturer, can't hurt on getting this thing spec'ed out. Whether or not those relationships will last once this gets rolling is the shaky part.
Don't get me wrong, the team seems like a great group of people, and I trust them to have good intentions. However, I don't give anything approaching trust to Broadcomm.
Roku has something with similar specs for $50 retail (which means they sell it to retailers at $30 at most). And Apple has something called "AppleTV", which has a little more hardware on it (wifi, power supply, ir remote receiver + remote, 8GB of flash), and sells for $90.
Amazing to think the impact of these low-cost computers.. goodbye expensive routers, NAS, set top boxes, PVRs, media centres. It can't be too long before they (ARM devices) start eating into the low end laptop market.