I've been using my own Mac at work for the last 4 years, at 3 different companies (granted, one of them was my own startup). The caveat is you have to support yourself, which is fine by me. I get to use the environment I'm most comfortable in and I don't have to deal with crappy Windows updates, virus or Outlook headaches.
I'm nearly in the same boat. I've been at the same company for some time now and started bringing my personal macbook to work to code on. After awhile, I was so productive they bought me a one for work use. However in a 1000+ person company I'm the only one on a Mac, but I also have to do my own support.
