I've been thinking through Ballmer's comments that he'll buy 20 Web 2.0 companies each year over the next five years, and a biting irony just hit me: Web 2.0 is all about collaboration and architecture of participation. Web 2.0 grows through community. Ballmer plans to get into this market by buying communities...
...which implies that he's not very good at building them. Now, some will cry "Foul!" given the rich partner ecosystem that Microsoft has grown over the years. But Microsoft's extant partner ecosystem is very different from the kind of community that open source and Web 2.0 fosters.
Perhaps instead of focusing on buying the web, Ballmer would do better to change his employee demographics such that Microsoft could build the web? This would require an open-source approach to community building. It would require Microsoft to be a very different company from the one that it is.
Even if Ballmer is able to bulldoze his way into the Web 2.0 market by acquisition, I'm not convinced that web communities are something that is susceptible to hostile takeover. Ballmer may end up with great properties...and no one to populate them.
In short, open source and Web 2.0 can't be forced. They're community-based, both of them. Microsoft must first learn community before it can effectively build or buy communities.