It already is. They've been laboring 5 years to get the Vista beast out the door, while Apple is coding circles around them releasing a major OS X upgrade every year. If Steve Jobs ever comes to his senses and allows OS X to be pre-installed on major PC brands (e.g., Dell), he could give the Gates empire a run for their money.
At this point, the chances of Jobs coming to his senses and releasing Mac OS X for non-Apple PCs are about the same as Linspire becoming the most popular OS of all time...somewhere between nil and nada.