I would say that the issue of support and whatnot depends on the environment. Clearly, an MBP will cost more than a typical PC (though, Macbooks, probably not). Also, if there are only a few non-Windows systems, it won't impact the software license budget much either.
Support should not be too much of an issue in most organizations, but it depends on the quality of your IT organization. If Microsoft technologies and methods predominante over standards and best practices, there could be the occasional problem, and it's also true if the business relies on platform-locked applications.
Platform support, however, ought not be a problem. My employer has 4500 people and is heavily invested in Microsoft product on the desktop (I think the license costs per basic system image is $2500 which includes the OS and various pieces of MS software plus the client licenses for services and whatnot). 1 in 11 (about 400) of the desktops are Macs, however. They employ one half-time person for Mac support, and 15 full-time people for Windows desktop support. The IT help-desk SLA for Macs is a response within 4 hours and for Windows PCs it's 24 hours. You can assume that most of the difference is that the Mac users are all in R&D, where one might expect the users to be more technically inclined.
In our case, the Windows support people are generally not technically competent in dealing with non-Windows technologies. In research, most of the technology is UNIX-based (be it Linux, or what have you), so IT generally doesn't service the research part of the business and is largely unaware of research IT resources and technologies. In that environment, there's essentially no difference between a Mac and everything else.
One thing that we have noted, however, is that access to corporate shares and resources is actually FAR more efficient on the Linux workstations than the Windows or Mac systems (given that the system is configured by a competent administrator).