Microsoft's PR has just sent out a message that there will be a major announcement
on March 15.
At this time no one knows what it is going to be or what it will concern to. Microsoft has no known new upcoming products with the exception of IE 9 which will be released this month; however, that isn't really a major announcement.
The only thing IMO that would be major announcement are Windows 8 which is too far away IMO, Microsoft tablet , new WP7 phones or a new Zune or the death of the Zune brand.