In my opinion it will sell to those gamers who dont want a PC/Windows based system. For me whos been gaming on Steam for 8+ years on Windows, its worthless to me. If i want to game in the lounge room ill just plug my tower next to the TV and play.
Unless they release HL3 on SteamBox as a timed Exclusive (which would screw over 50+million of there supporters) i cant see many Steam owners buying one.