"Virgís a little off, because now that Macs are Intel-based, you get "hackintoshes" like my SOís where they build what is essentially a PC, but runs OSX natively. Itís just not manufactured by Apple. Although one could argue that itís not a Mac in that case, and I guess I agree."
That's it exactly. A computer isn't a Macintosh just because it runs OSX, because that's a brand name for Apple's line of computers. As for Frankentoshes, I figure anyone who's savvy enough to build a PC-based machine that can run OSX will know what they're running without resorting to looking over the keyboard or case.
Also, not every PC runs Windows. A Chromebook running Linux is based on the chipset and board architecture popularized by IBM for their "PC" brand, so even though it's not Windows it can reasonably be called a PC (although to be completely technically accurate it's a PC clone unless it's built by IBM/Lenovo). A Macintosh these days is rather close to being a PC clone as well, but the difference in board architecture is enough that it's a stretch to call it a PC for now.
Now I agree that my rule isn't perfect, because there's no Apple logo on machines like Sun or SPARC stations that can't be called PCs, but again someone running such high-end hardware is unlikely to need a cheat to figure out what they're using so as a simple rule of thumb it works.