I work at a large internet company. We get to pick either a mac laptop or a windows laptop, plus a redhat desktop to run our code on. The vast majority of new hires go with the mac.
But, we are generally writing php or c++ and many, if not most, use vim or emacs. I would say developers now at my company are about 30% mac, 50% windows and 20% linux. Our production machines mostly run linux. I think the mac is a great development platform, because it runs office, vim, textmate and I can and do run windows in vmware.