It's interesting that some people seem to be treating Steam Machines in the same way as they would a PC, rather than treat it as a console, and bringing up all sorts of negatives as if they were the target audience when at best they may be a small part of it. I hope Valve are very successful, I think it would be great to have a situation where (for the most part) "console" and "PC" are the same thing and there is no need to port anything as they are the same platform with consoles at worst being like an old PC that games companies try (perhaps a little too hard) to keep supporting ahead of bleeding edge hardware that few have.
Also, with PS4 and XBone being fancy PC hardware (what I presume is a standard x86 CPU instruction set being the key here) they are both potential Steam Machines, the work of porting SteamOS to either platform (Sony/Microsoft permitting) would be fairly easy and nothing would need to be done to the games for it to work. Sony allowing dual boot is a remote possibility I believe, Microsoft is likely to take an "over our dead bodies" approach.
edit: the Heterogeneous Unified Memory Access architecture of the consoles would likely complicate SteamOS porting matters somewhat but if the APIs such as OpenGL were suitably modified on the hypothetical PS4/XBone SteamOS ports then while it wouldn't necessarily take advantage of the different memory architecture it would still allow those consoles to behave like a standard Steam Machine from the point of view of the games and end users (assuming there isn't something else to get in the way of a port and that HUMA isn't an insurmountable obstacle).