The percentage of home computers that are Macs in America should be much higher than the overall market share. Windows' market share is made up far more of business users and embedded systems than the Mac OS's. Also, market share numbers are calculated by computers sold, and Macs tend to stay in service longer than PCs, so that also skews the numbers. I wouldn't be surprised if the actual user base in America is closer to 10%, perhaps even higher among home users.
There are no good statistics on the matter, though, at least not that I'm aware of. Just market share, which doesn't give a complete picture at all.