If I was Apple I wouldn’t be worried at all about Windows, I would be worried about the next generation of Linux desktop software. The main reason I’m considering a G5 for my next desktop purchase is that I want a powerful machine that Just Works when I plug stuff in and can still run all the open source tools like Subversion, rysync, PHP, MySQL, etc etc that I rely on. It’s also interesting that all the software I regard these days as truly essential isn’t desktop software, it’s server software. I can survive switching text editors or graphics programs or even operating systems, but if I had to use ASP and SQL Server instead of Perl/PHP/Python and MySQL I’m not sure what I would do. I can function without these things on my desktop, but having to access them remotely (if it’s pretty transparent) prohibits some pretty cool stuff and diminishes my productivity.
Windows and OS X are tools I use to get things done. Linux desktop software (X, KDE, Gnome, etc) is a hobby. If I could focus on getting work done instead of getting my wireless card to work I could consider as a serious and cheaper alternative to a OS X desktop. (No matter what I want one of those new Cinema displays though.)
To preemptively clarify, my comments do not at all apply to Linux in the server space, where it by far the most mature and capable platform out there and I would hardly consider anything else.