I run Win7 in a virtual machine on OSX for .Net development, since I don't need hardware layer specifics it works great. If I were building applications for the desktop I wouldn't likely it do it this way, but for Web dev it works great.

I installed them as dual boot at first but all the driver issues started getting annoying. So I created a VM and it works great now, plus I dropped a couple Linux distros on there as well.