Should there be a difference? My proposal
[mac]relates to hardware
[osx]relates to the software
[apple]relates to the company specifically.
I agree with KeithB, [apple] is very vague to what it actually refers to in any given situation. [apple inc] is kinda strange, but still better. [mac] is related to hardware questions about all macs. We should of course have [macmini], [imac], etc. for different models. [osx] is regarding the operating system and it's built in functionality, excluding standalone applications like AddressBook, Safari, iCal etc.
As a side note, how about [snow-leopard], [tiger] etc? Is it better or worse than [osx 10.6], [osx 10.5]. And is it possible to alias tags? Like the aforementioned [tiger] <-> [osx 10.4].
Absolutely, I agree Macintosh is just a brand of hardware made by Apple. That's the way of reality.
What's wrong with using the real-world identifiers? How about: [apple] is the name of the company, [macintosh] or [mac] is the name of the computer line, [ios] is software that runs on the i-devices such as iPhones, iPods and iPads, [macos] or [osx] is the software that runs on the Macintoshes.
This is how it is in the real world and not very complicated at all. You can use multiple tags anyway.