First Kai-Fu Lee, then Marc Lucovesky, then Robert Scoble, then Martin Taylor, and now Vic Gundotra have announced that they're leaving Microsoft.  All except for Scoble (and Taylor, yet?) are going to Google.  This exodus has set the blogosphere abuzz about the reasons why this might be happening.  My guess is, it's one of the following reasons:

1. It's a coincidence.  It's only five people, a very small percentage of the over 70,000 people who work for Microsoft.  My money's on this horse.
2. Microsoft's vision has stopped resonating with it's employees, and the ones who have the ability to jump to another high-profile company are doing so.
3. Microsoft has become too beaurocratic with too many layers of management for visionary employees to make a difference.  This seems to be a common complaint from Microsoft employees.  Hopefully new execs such as Ray Ozzie will be able to fix this.
4. Microsoft isn't paying well enough to keep top talent.  I doubt it.  With billions in the bank, Microsoft can afford to give important employees rides to work in personal jets while getting full body massages and washing down caviar with Dom Perignon.
5. High-profile employees have more fulfilling opportunities available.  I'm pretty sure this is why Scoble left, as he's basically starting his own company now.  There's no better feeling than working for yourself.  I'm not sure what, if anything, Microsoft can do to prevent this.

Guess we'll just have to wait and see.