It depends how you define significant. Gates has been selling stock and has reduced his shareholding from at least 45% to around 2.5%. While $11.3bn would be a lot for most people, I imagine things look different when you're worth $86bn to $90bn ;-)
Incidentally, Yahoo Finance reckons The Vanguard Group owns 525,395,707 Microsoft shares. The Vanguard Total Stock Market Index Fund is just one of its funds.
YF doesn't mention Ballmer so presumably he sold off his 333 million shares without it making news. It's a while since I looked....