If SteamOS lets me play AAA games (and not just Portal, Half-Life, Left4Dead and Team Fortress) AND it lets me run XBMC or Plex or some other TV software, then I'd switch in a second.
I've been very happy with the increasing options on Mac OS X and I hope they keep improving.
I'm way more excited about slower, turn-based games, and Civ V, Europa Universalis, and FTL have been enough to keep me from searching for anything else.
Despite this, I know I've been missing out on games I would probably love. I think I see a SteamOS device in my future.
SteamOS will be great for linux, but people seem to assume Valve are taking a very exclusive, traditional console approach, when they're trying something a bit different.
I also assume that basically any Steam game that comes native to Linuxt will also come to Mac OS X.