One of the core Subversion developers, Garrett Rooney, posted an interesting piece entitled What makes an Open Source Company?
I agree with what he wrote, so I do not have a lot of commentary to add. My definition of an open source company would primarily be based on how the company interacts with and supports open source software development. Do they have developers that significantly contribute to one or more projects? Do they engage the community, or at least allow their developers to engage the community? For me, those are the determining factors.
By the way, Garrett was the author, now co-author with Daniel Berlin, of one of the best Subversion books available: Practical Subversion.