It probably wouldn't be worth it for them. MS wouldn't give ground easily, and it isn't clear what they could do that either the Linux guys or Apple havn't. What they might well try, though, is gradually expanding their collection of rather cute browser based apps. The Google people seem to have the scripting stuff down(e.g. Google maps, or that GoogleX thing that has been floating around, and Gmail, of course). This gives them most of the advantages of a Google OS, namely serious brand recognition and everyone, everywhere, depending on them all the time; without the hassle of writing device drivers for every last proprietary horror, Tiawanese cheapie, and broken-as-designed-winwidget out there. They might do something like slapping together an installer bundle with Google Desktop Search + custom version of Firefox with loads of Google web apps integrated into it(they've hired some important chap from Mozilla, if memory serves) + Thunderbird designed to jack right into your Gmail(directly, with the same interface, not POP3). Doing an actual OS, though, is a fairly thankless task. Much, much easier to just use web based stuff that will run on top of pretty much anything; then you're set no matter who wins the architecture wars, the OS wars, etc.
