I've pretty much always been using Windows (on the desktop), and never found a compelling reason to move to another operating system.
That said, over the past few years I have noticed an increasing number of MAC OS X notebooks present at PHP conferences, which intrigued me and has kept me interested in where the MACs are heading.
No doubt that there are some compelling reasons to move to a MAC.
For me as a developer, one of the biggest reasons would be having a UNIX development environment natively side-by-side to the desktop environment. Although VMWare is an amazing tool, and I'm a big fan of it, booting and running it, is not always natural, and half the time I actually prefer to SSH into a remote server on the Internet as opposed to powering up VMWare.
Another compelling reason for the MAC is the slick GUI. I have always been especially jelous of the cool F9 Expose feature. Yeah you could say it's a minor detail, but when you have lots of apps running, alt-tab is a real headache, especially when some of those apps are the same (like many firefox instances). I finally found a cure for that called WinPlosion (http://www.winplosion.com/). Although it's not quite as sexy as the MAC solution, it's pretty cool, very lightweight and slick. If you're a Windows user, give it a try.
So where does that leave me? I guess for me personally, the achilles heel of the MACT is that there's no chance in the world that I'll ever get used to a TouchPad. I just can't deal with it. The Thinkpad TrackPoint (a.k.a nipple) is probably the main reason why I'll continue using Thinkpad's. If/when Apple ever adopts it, then I believe I'll be ready to make the move. And as at that time, MACs will be running on x86, I believe there'll be even more of a reason to make the move.