There are advantages and disadvantages to both a mixed environment and a mono culture. Personally, I prefer a mixed environment within reason. I feel it has less security problems and more flexibility. That said, I'm not for giving every user an option on what OS to use.
My company has a mixed environment. We have a smallish network of only about 1500 computers, so management isn't a big deal. We use novell to handle drive shares and permissions. All new computers get a nice image dropped on them and then deployed. All users are restricted except in very extreme circumstances where some windows applications simply require it. Even IT users run restricted profiles. But honestly, managing macs isn't very hard. We have about 100 of them on our network. I can push apps, printers, etc. There is no security policies to be concerned with (restricted meets every need I've had set down), etc.
I'm all for using the best tool for the job. If someone has a legitimate reason for wanting to use any OS, then I will help them put together the paperwork and take it to my boss for consideration. I myself no longer have a windows machine at work. My productivity has increased in most areas because of this. I spend 40% of my time using nothing but a terminal, 50% of my time doing web development. Textmate, and now xcode has been a huge performance increase for me. I have only one app that still requires windows that I use and we are in the process of ditching that product (for unrelated reasons) and replacing it with one that is linux/mac/win friendly.
So who gets macs at my company?
1) Web developers - programmers are basically given anything they want within reason and left to themselves for the most part. They submit a req and it gets bought and installed. All of us have moved to macs from windows or linux and have found the process to be much nicer for us.
2) Marketing - Most of the marketing people we hire have tons of mac experience. They know how to use them and it is better to keep them using what they are happy with.
3) Our mac lab instructors - makes senses right? The people who teach mac relating stuff at the college get macs.
4) Anyone with the word Dean, Director, or Vice, or president in their name that asks.
5) Anyone who has someone with the word Dean, Director, or Vice, or president in their name ask for them.
6) Anyone who got the inclusion of a mac rolled into their grant.
Everyone else gets a dell with windows XP on it. The exception being the guys who teach linux, they get linux (duh), and our network/sys admins who also use linux.
We don't manage linux the same way we manage osx. The guys who are allowed to use linux are at a level where they are just given a notebook/desktop and a free license to do anything they want. This would never happen to a normal user.
What kind of restrictions do you typically enforce on your users? I've worked for a few large companies and it seemed the only thing we really enforced was that you couldn't install unapproved software.