[david_shaw]

The only real security concern that I can see is that if you become infected with some sort of malware (or you get hacked), there may be no other noticeable activity on the network. So, you might say "my computer seems a little slow today," but since the malware wouldn't be infecting the Windows computers, there'd be no alerts on their IDS.

The preceding scenario is a ridiculously contrived and unlikely event.

The truth is that the BSD kernel underneath OS X has a lot more security credibility than does Windows. Furthermore, since OS X has a lower market share than Windows, it is less likely that malware creators will target it. This does not make you immune from hacking, but since you are directly comparing your situation with Windows alternatives, I think it's fair.

Additionally, OS X these days is very inter-operable, so I wouldn't really worry about IT compatibility too much.

The biggest concern I'd have is convincing them that it's worth triple the cost of its Dell counterpart. That one's a little more tricky.