AOL, and before them CS, and others before have tried to capture the audience. If anyone has come close it's AOL. (There are millions of "family computer user types" who don't know anything exists beyond the swirling "A").
MicroSoft could more easily open up client server versions of the office programs that "everyone" already knows and uses and roll it in for MSN.
There will still be people that will swear by their favorite non-MS solution, but the market share they already have makes them the obvious ones to pursue this environment.
You can bet dollars to donuts, (sorry --- I don't know if that idiom makes sense anymore now that donuts can cost a dollar or more each), but anyway --- you can bet MS will develop anything like this 6 months to a year AFTER anyone else does it --- it's their way... as soon as someone opens a new category of computing, they give away a MS-Category-Killer v1.0 and keep giving it away... then when there's no competition, they will own 100% end-to-end computing and user data for the entire planet. (I got carried away, sorry).
Back to the G story --- the Ajax talk is intersting, but in the end it's HTML which does not make for an OS. If G did develop a client OS coupled with a 'terminal/device' with; removable storage; server data file management (including "delete"); data synchronization, (when CLIENT requests it) ...and you've got something.