Real world market share matters nothing, we are talking about law school here. You will use this computer for a 3-4 years while in school, and by the time you get out, you will need to buy a new one anyways (any OS your heart desires). Get the laptop that is most compatible, a pc, and just make sure you don't open weird emails. I mean, who has time to do anything but type and internet research on these things anyways? Any computer today can do that without much trouble.
If only resisting the urge to open weird emails were all it took to keep a Windows machine clean... I'm a tech guy at a large law firm. I spend hours every week cleaning spyware off of Windows-based computers. Every now and then we get hit with a virus and I spend the better part of a day running around removing from it from some of our computers (despite multiple layers of anti-virus software).
It seems to me that the only real compatibility issues are caused by exam software and possibly the limits of your schools tech support. TCP/IP networking certianly isn't limited to Windows, and even Windows-specific networking constructs (e.g., Win2k domains, Active Directory) are supported in Mac OSX. I'd say get the computer you want, whether it's a Mac or a Windows-based machine.
On the other hand, if you buy a Mac, don't count on using it at any law firm large enough to standardize on hardware and a software image.