Well Longhorn is a long way off and the 'just stop the kids from downloading crap' argument dosn't hold so I think we'll see a lot of people migrating in one form or another.
Doubt it'll be Linux in its various guises or Solaris that gets the increased market share, which I foresee as almost inevitable given the epidemic status of spyware and virusses (which I note is only likely to get worse). Linux is too fragmented in its champions, as shown by various threads on here, to bridge the acceptance gap.
Macs on the other hand are well placed and seem to have shifted their business model from hardware / software vendor to supplying services and cool stuff. At the end of the day this is what peeps want, hassle free computing and accepting small charges for copyrighted or exceptional material.