1st hint has been given for the next game: It's on Xbox Live Indie Games.
So, its only windows
XBLIG are usually made for computer (although sometimes with XNA) and ported to the console - I hope I'm right, I don't really know what I'm talking about. It is just one of the outlets for Indie devs, however, I feel that Desura/IGS is the way to go. Anyway, point is, could be on any OS... guess we'll see.
Well, basically, XNA is working on top of NET that making it Windows oriented. These games typically easily (with some quirks like only XBOX controller support) ported ot rather compiled for Windows, but nor running then not porting to other Oses is not easy. Because of that i'm kinda not impressed, since XNA games in most cases looks and feels pretty cheap (not always, but often) and stereotyped. Well, IF its XNA...it may be not.