Beyond the differences between community and sponsored projects, if it is true that buy an existing open source community is not easy (yet possible), retaining a sponsored one seems difficult looking at the Citrix versus the Xen Community case.
Wet open source ground by rupertsanfordscutt
Daniel Kusneztky reports that XenSource, struggling to find ways to appropriate returns from the commons, is embarking in a new direction, potentially harmful to other members of the XenSource community.
Sun Tzu said there are six kinds of terrain, open source communities are an entangling ground: ground which can be abandoned but is hard to re-occupy. Traditional IT vendors buying small open source companies have to mind their steps, open source is a different ground, definitely.