Microsoft is the entrenched vendor but, as a result of delaying Vista and failing to build much of an IT wave for it, they have left the door open for significant competitive challenges in a market they have had locked up for some time. If they can get Windows 2010 out in time, and it meets the expectations that Vista didnt, then they likely can retain dominance even if they do lose a lot of market share. But, if they dont execute, the non-Windows alternatives could turn them into old news and that would be a lousy way for them to end the decade.
For you, the good news is you will increasingly have alternatives, and that is the bad news as well, because choices do make life both more exciting and riskier. And because the missed dependencies (custom apps, unique hardware) and there are always missed dependencies have a tendency to really take the fun out of any platform change even if it is to a new version of Windows. But, you are going to have to move, so youd better start setting up to make as good a choice as you can when the opportunity presents itself.
I suggest you watch all three options and then follow your head to the right decision for your company.