Valve is shipping the first 300 units to selected beta participants living within the United States. Accepted beta participants are being announced this afternoon. Valve also shared that their Linux-based SteamOS images will be available when the prototype hardware shipps -- Friday. It will be available to download by individual users and commercial OEMs.
All the exciting details can be found via this SteamCommunity.com posting. I'll be working extra hard this weekend testing out SteamOS prior to leaving on Monday.