Sounds like it'll just be a specific PC that Steam can whack a label on their games to say "Steam Box compatible" rather than "requires X amount of RAM and X processor and blah blah other specs". If it isn't just a rumour, I think the intention of this is not to make a standard for developers to develop for, but a system that steam can say "X game will definitely work on Steam Box", ultimately simplifying the process for "simpler-minded" end-users, and possibly expanding the PC games market by attracting console gamers. If it catches on massively, which, while I doubt it will, is a possibility, Steam essentially become a major player in the console war, as developers start to develop specifically for the Steam Box. If it doesn't catch on, then it's no biggy really, but there's no real disadvantage to it.
Could potentially be a good thing, but will probably just slip under the radar, I think.