It's definitely a 'modern' update to the logo, ala the recent Wal-Mart redesign, which ditched their sharp corners for a more rounded look, and replacing the star with a nice, soft asterisk. Kraft tried something similar... but it wasn't too popular. While Microsoft's logo still has plenty of corners, the bright colors and the thinner font is more friendly than their previous logo and fits with the current trends. The change comes just in time for a big product roll-out, so you'll be seeing this mark more and more in the coming months.
I think it's a good move. While Microsoft may have been using the same logo for the past 25 years, I would guess that most people associate their name more with their Windows proucts and with the 4-color window mark. Many companies contemplating a logo change need to be worried about losing recognition, especially after such a long time, but in Microsoft's case I think it actually helps them out - because of Windows' success, that color combination in that layout instantly screamed "Microsoft" before they even made this change.
Some people feel like the logo is uninspired and boring, and while I won't argue those points, I still think it works for them. Sure, as a designer I love to say "I could do that," or more commonly, "I could do that better," but does it need to be? Look at their competition: Apple, the masters of beautiful simplicity. This new logo helps to move Microsoft in that direction (though we'll see if their products do the same) and finally out of the 80's.
What are your thoughts on the new look?