The relationship btw Dell and Microsoft is a very interesting one. Michael Robertson, of Linspire, has a little article on it in his latest "Michael's Minutes".
Towards the end of my time at b-school Kevin Rollins visited the GSB. Good speaker and very nice guy. Afterwards in the q&a section I asked him what he thought about selling more Linux machines and how that would affect their relationship with Microsoft. He basically responded that it did not affect them in any way - Microsoft sells products to Dell's competitors and they were free to do the same. But as we can see from Robertson's writing, Dell is probably still constrained in this area. Its interesting because Dell has grown into an enormous and powerful company in their own right, but as Robertson mentioned, they have little flexibility in their business model.