With Apple selling the next Mac OS upgrade at $29, Microsoft is going to have seriously rethink its consumer pricing structure, if only to save face.
That's an upgrade price though, and IIRC only from the most recent version of OS X - how much does it cost altogether?
Also it's irrelevant for 90+% of the market, who aren't going to rush out and buy a Mac (and even if they did, the cost of switching would be much higher). From a matter of principle, we might as well say that AmigaOS only costs £30 or whatever it is
A better comparison would be pointing out that Linux is free, which is a concern for them on netbooks, although Linux has been around and free for years, and hasn't put a great dent in Windows's market share.