How many is irrelevant, the point is you can be open without compromising other features and it only helps you. Certainly, despite a large selection of hardware with mostly equivalent features I can't be the only one who jumped on the N900 because of its software stack.
Depends on how hard Intel pushes it, I suspect.
Nokia's already a given. The biggest potential drag is Android, and the biggest benefit is MeeGo's proximity to the rest of the Kernel community.