Look at my mail headers, Doug -- I succumbed to the Apple bug a while ago ;-). It helps that it now runs BSD underneath.
What am I getting at... simply that while I am as happy as the next person in indulging in a bit of Microsoft bashing, we need to reserve such things for the happy hour, not serious discussion (IMHO, of course). Jordan et al are quite right, AFAIK, about Microsoft's focus, the way it treats software, etc, but nevertheless, they are not to be equated to the big-3 of US auto. For one thing, they still own the industry. But more to my point, they still put out software that is reliable, inexpensive, easy to use, works on a range of systems, integrates both within a system and with external services, etc, etc.
While you and I might enjoy our slick OS X UI, that is indeed to each our own (my mom, for example, finds the Windows interface much more consistent and easy-to-use; and anyone with RSS/CTS/etc will probably have little praise for the OS X's required gymnastics for UI operations like exiting an application or bringing up a context menu). There are some things a Mac indeed does better - sync'ing my AddressBook to my bluetooth cellphone using iSync was effortless. There are other things it just doesn't do at all. These other things also happen to be fairly important stuff (for instance: integration with network services infrastructure... if that sounds techno-jargon, think for example, "calendar server"). In my opinion, important enough to leave Mac in its current niche segment.
In short, Microsoft (with all its ugliness, silly imagery regarding Gates and his technical prowess, etc) dominates the software world at least as much because it just does a better job of it, taken as a whole, as because it is leveraging an ill-begotten monopoly.
--ravi