I believe "PC" has historically meant (or often been used to imply) "IBM PC compatible" (http://en.wikipedia.org/wiki/IBM_PC_compatible) which Apple/Mac was not, until they switched to x86.

The problem is that viruses don't relate to the architecture - otherwise linux and *bsd (on the x86 platform) would have also been windows-like with their malware.

I know I'm playing semantic games here, but so is Apple with this slogan :)

In the context of Apple's "Mac vs PC" campaign, does anyone seriously have doubt about what they mean by a PC?

I know I'm playing semantic games here, but so is Apple with this slogan :)

Not really - in context it's perfectly clear what they meant.

In the general public's eyes Macs are computers, so yes they are PCs. In the Appleland, "PC" is synonomous with "Microsoft Windows", and hence Apple Macs aren't PCs.

I had a long time Apple user ask me if I had a Mac or PC. I was using Ubuntu Linux at the time, so I said PC :P

You should have just said, "Linux". Mac users make this distinction because they constantly need to inform I.T. manager's that they are not using a Microsoft Windows OS. Originally, this meant an IBM-PC or an IBM-PC compatible computer, but it was a whole lot easier to just say "PC". I'm sure Linux users also need to explain to their network admins that they're running Linux.

The term "PC" persists not just for historical reasons, but because its hard to come up with a replacement term. A "Mac" refers both to hardware and the OS, whereas a "PC" means "Windows OS running on Windows-compatible computer". I suppose we could replace the term "PC" with "WOS-ROWCC".