During a lecture at the University of Texas, Gabe Newell mentioned that the Steam Box's biggest threat isn't the home consoles, but instead Apple.
The biggest challenge, I don't think is from the consoles. I think the biggest challenge is that Apple moves on the living room before the PC industry sort of gets its act together.
Newell says to expect a lot of living room products that will take advantage of the PC you already own, not unlike Valve's upcoming Steam Box. It will definitely be interesting to see if Apple capitalizes on the living room, considering, as Newell mentioned, their "huge amount of market share."