This can be a very good thing. A lot of people are fed up with Windows and hungry for something better. If Mac OS X can compete with Windows on the same hardware, I’m sure a lot of people would switch. It could be a real threat to Windows when people try it and find that it actually is better, and they don’t have to buy a new machine to run it.
Apple will most likely prevent it from running on standard PCs, but that’s very short-sighted. As long as people will have to buy a new machine to run OS X, its market share will always be limited. A good strategy for Apple would be to build the same elegant hardware they’re building now and sell it at a slight premium over standard PCs. At the same time they can sell OS X to other PC users. The people who appreciate the elegance of Apple hardware will continue to buy Macs and it may win over some new users. At the same time, the masses who just want a cheap PC but are disgusted with Windows, will buy OS X to run on their PCs. When people see that OS X is superior to Windows on the same hardware, it will seriously cut into Windows’ market share.