In many cases, linux is _not_ a good replacement for OS X. The case, of course, being that you actually _want_ to run OS X.
... I guess for me the discussion is really one of three possible cases:
1.) If you want to play around with OS X to see whether it's worth what all those Mac geeks are constantly bragging on about, installing OS X illegally on a PC might seem like a good (because free) option. You could also go to an Apple store (or other store selling Macs) to try them. Of course, if you're playing with a crippled Mac at a store for five minutes, you won't get a really good impression of what a Mac's capable of. I think for _this_ case, Apple should actually create a demo-DVD of Mac OS X which _does_ run on many PCs. It's perfectly doable, but there's of course, a danger that this very disk could lead to easier OS X hacking...
2.) You _want_ to switch to the Mac, you know it, but the hardware's too expensive for you, and you'd also need to buy a lot of software (cross-grades, sometimes...), which further makes the switch more expensive. Well: I guess there's only one choice here... Get the money to do so.
3.) You have no intention of ever buying a Mac or related software and just want to be a free-loader. So you download illegal copies of system & software. Well: You're a criminal.
But the interesting case, in my opinion, is of course number one... Apple could reach many, many people. Put a demo DVD of Mac OS X (live system?) in every iPod's packaging. Wouldn't _that_ be a great way of reaching people. I know: Many PCs out there probably don't have the hardware to run if flawlessly, but you could make it clear what the requirements are. Easy. Good marketing. There could even be demo versions of MS Office for the Mac as well as demo versions of Adobe's and Macromedia's software inside... (once they're universal, I'd say, since else they don't make a really good impression...)