Sorry Microsoft, we're not interested.
During an interview with Golem.de, Valve Engineer Ben Krasnow confirmed that Valve's Steam Box (working title) will not be Windows-based and will instead opt to run on a Linux backbone. As big of a move as it is, it's not surprising given Gabe Newell's long-shared disposition with Microsoft. Although he worked at the company before founding Valve, he doesn't agree with many of its practices and has even gone on record to say Windows 8 is "a kind of catastrophe"—which it has been so far, for the record.
In addition to the Linux news, Krasnow also cemented the console's release for this year, with other big detail reveals to come between now and its launch day. Although no launch date was specified, it was hinted that the console's controller, hardware, and other "cool things" would be revealed soon.
Although Linux has been a standard for servers, it has virtually no support from gaming developers. Valve has some work on its hands to prove to developers that it's worth the investment. Otherwise, the Steam Box might be stuck playing only Valve games, and we all know Gabe Newell is more ambitious than that.