In my mind the debate should really be OSX vs. Windows XP at this point...not "mac vs. pc".
They both have respectable hardware and a lot of it overlaps both platforms.
Someone mentioned Linux and Unix etc...which is proof if there ever was that "pcs can cut it". The problems with stability and such that everyone complains about are rarely hardware related. Though in some cases as this thread has pointed out...it can be bad hardware, bad setup. Etc.
Mac OSX was built from the development of NextSTEP - which took it's foundation in BSD(Berkley System Designs)4.4-lite (the last official BSD release that was a legal separation from AT&T UNIX systems).
That operating system (which finalized development in the early 90s) is one of the most stable and efficient of all times...and it's TCP/IP stack has propogated almost every computer out there today. FreeBSD was built on it - and runs about 80% of the webservers in the world.
Virginia Techs "X System" - 1200 G5's in a cluster, is ranked as the 7th fastest super computer in the world. (behind computers from IBM and others using Linux clusters).
Obviously, MacOSX has a huge advantage over Windows XP (even though WinXP *can* work). Only time will tell if Longhorn is going to be worthwhile for audio recording. I have my doubts.
The problem with Linux/BSD on a DAW PC is drivers - and of course also recording software...though more does exist in that area than drivers.
I've yet to find anyone who has a good multichannel soundcard working in that environment.
If Longhorn is dissapointing and tides keep changing from Windows though, manufacturers may decide that supporting Linux or other open source projects is important (like nvidia and ati have). That would lead to an increased interest in developing more state of the art software for recording on the platform.
Just for reference, there is a program for Linux called Cinellera for video editing, that many in the industry claim rivals final cut.
So yes, they are very viable platforms...and hopefully where the future of personal computing goes. But they aren't there yet.
That being said, if you follow what I was saying at the top of this post - you could consider Mac OSX as almost a "glimpse into the future" of operating systems.
(if this kind of thing interests you, check out the history of UNIX on wikipedia, and google for some information on the history of NeXt - the world wide web, and Doom were both developed on that platform. Pretty neat)