The thing about the macs is that MS can count on apple to screw up or at least manage to piss off the corporate world to the extend that they wont touch the macs.
With linux on the other hand were now seing companies like dell HP Novell and IBM starting to endorse linux in some areas and those companies have street cred in the uptown suburban areas where corporate ececutive types hang out this is wry MS at every instant have gone after linux much more aggresively then mac.
MS is the king of the gaming PC thats noone will take away from them but what about the internet browser? or the corporate desktop client in a world where everything shuts down when the network connection dies? What happens when the intranet replaces microsoft word templates as the tool of choice for every business process?
When it was just IBM novel and a few mid range server operators ms did not care today it's desktop hardware companies experimenting with linux on the desktop, along with an general tendency to move the critical app back onto the mainframes thats stating to apear. MS might be forced out of their current position, and into one wheir they have to fight for even a 33% market share.