IMO, I would say that there is a world of difference between what is "Supported" and what is "Allowed". The company has every right to dictate what they want to be responsible for supporting as well as determining what they want to allow on their network, particularly from a security standpoint.
If you're a primarily Windows shop, but you have people who want to use Linux or Mac, and they can competently support themselves securely, then I would say that so long as they keep the patches up-to-date (and possibly running anti-virus - No, this isn't an invite to get into that discussion), then I would say that they might be allowed to run as they wish. But don't expect much in the way of support from the IT dept other than connectivity and hardware.
The same goes for other apps. Perhaps the company standard is IE, but you have some diehard Firefox or Chrome users. If they can keep themselves up-to-date, then that's fine, but don't expect them to get support from IT.
(Ditto for Blackberries/iPhones/Android/WebOS/etc)
Then again, there is the reasonable case for 'Our Network, Our Rules' policy. Every component does have the potential to impact every other component...
If you are able to make the difference known to your user community, AND make this understood by the Execs (and the IT management doesn't get bullied because the other execs want their "toy"), then you should be fine.
Note that this in no way takes into account Security/Regulatory policies (HIPAA, SAS70, SARBOX, ISOxxxx, etc) - they may impact what is legally allowed.