Good question, I went to visit my local hifi expert/shop guru to ask the very same thing, he basically said that mac have designed the audio end of ther machines from the ground up.
The optical output is directly wired into the harddrive and does not get routed through the soundcard in the same way that other PC's work, therefore the data stream is completely pure with absolutley no influence from third party software, also the apple lossless codec was specifically written with high end audio in mind and specifically for Meridian, so it is the best of the codecs as far as Hifidelity repro is concerned.
He said that he has heard many PC based servers in very high end setups and the only one they recommend in the shop is mac.
Nice guy with a tech background and a lot of experience running a very high end shop, he certainly sounded convincing, after a fair bit of research i'm headed down the Mac road, just got a 2nd hand one on
eBay and pumping about 1000 cd's into it like theres no tomorrow.....