I think one main appeal of steambox that some of you are missing is that custom high end desktop PCs are huge and ugly. my fiancee hates having my PC in the living room and I kinda wanna get it out of there too as its a bit of an eyesore. I'd be happy to replace that giant pc with something much smaller
You can easily build something that's smaller, yet powerful. I'm not really in the loop with PC components anymore, so I'm not exactly sure of the cost of such an endeavor. But on the software front, you could easily mimic a console experience by forcing your Windows PC to log in to a profile automatically, using
XBMC Launcher to have explorer.exe automatically disabled and instead of launching XBMC, reroute it to a batch file that launches Steam in Big Picture mode. So you'll basically have a computer that boots into Big Picture mode upon starting the computer. Get a wireless receiver + Xbox 360 controller and you're set. I'm clueless when it comes to Linux though, but I'm sure the same basic setup could be achieved.
I'm definitely interested in these Android devices though. They seem be fairly attractive in the form factor department, and while I don't PC game as much as I used to, it's potential usage as an XBMC device is definitely appealing.