The writing is already on the wall; the days of the Windows hegemony are numbered.
That's not to say that you'll see either Mac or Linux marketshare pass that of Windows anytime soon, but that the forces that may very well eventually lead to that are already hard at work.
Witness the lukewarm reception with which the world has greeted the release of Windows Vista, the successor to Windows XP, which was itself released half a decade ago, and compare that with the anxious anticipation prompted by the impending release of Mac OS X 10.5 (Leopard), the successor to 10.4 (Tiger), which was itself released only two years ago.
The plain fact is that Windows's rivals don't have to grab >50% marketshare to drive it into the ground; they merely have to apply enough pressure to make further development unprofitable for Microsoft, and that may already have happened. Microsoft spent a huge amount of money on Vista, and may not have anything to show for it at the end of the day.
Granted that Vista Forever is likely to last quite a bit longer than some other abandonware has, with continued updates to important applications for years to come, but sooner or later it has to grind to a halt, and hanging on as the market moves on to something else is sure to become an increasingly expensive proposition.
Is it time to jump ship? Well, if you're heavily invested in Windows software, maybe not, but it probably is time to begin to consider other options and, where possible, reduce the degree to which you're locked in to Windows.
If you're not heavily invested in Windows software, then now is as good a time as any! By most accounts, Mac OS X is a superior operating system, and several Linux distributions at least merit a serious look. Also, the advantage that Windows once enjoyed in the availability of application software is fast dwindling, as more and more publishers come out with Mac and/or Linux versions.
For people working in IT, it's high time to consider whether you might not be too young to spend the rest of your career patching Vista.
Think about it.