Actually, let's not do this. Let's realize that Microsoft is a company and Apple is a company, and while yes, in some fields they are competitors, let's just put the whole Apple vs. PC idea to bed. Guess what: a Mac actually is a PC. It's a very, very well-made PC (in fact, the best made, in this blog's humble opinion). And why wouldn't the folks at Microsoft want to use the best PC out there to develop and run their programs?
The battle between Mac and PC was cute back during the PowerPC age, when there were major differences between the two platforms. But things have changed: Macs run Windows, dogs and cats are lol-ing together, and the war's just not worth fighting any more. Of course Microsoft employees use Macs -- who wouldn't?