For some years I was the CEO of a services business that did most of its day to day trade through the web via a customer portal. having previously been a Mac user I used my Mac powerbook in the windows environment and converted the sales director to Mac's as well.
My IT people were resistant. One issue they repeatedly raised was that they could not run their antivirus software on the Mac's and that security may be compromised. Bearing in mind that this was at director level, it was a reasonable concern. I was never sure how much of an issue it really was.
The Mac's functioned well in the Windows environment and sharing of Word. Excel and Powerpoint files was never an issue.
Both the sales director and I used Entourage for email. It is not as good as fully functioned Outlook, but probably better that outlook express. Certainly our use of Entourage caused issues with on line diary management for our secretaries. I rarely use Entourage now, as I use Mac Mail. That is not brilliant either from a business perspective.
We also had problems connecting to the VPN, especially from home.
Some of our business software, particularly the sales and inventory management tools (that were partially accessible to customers as well as staff) were not Mac compatible and this was a nuisance for the sales director.
We also had an issue with back-ups. All of the networked windows machines were backed up on a scheduled basis automatically. This was not possible with the Mac's within the same framework and we had to take additional steps.
In truth, if I were a CEO of a business and had one of my staff wishing to use a mac when the rest of the business was windows, I would probably say no. I think that for the business, it tends to cause costly hassle (because of wasted time) if one's IT guys are not Mac familiar.