Not true. Nowhere in the agreement does it discern between physical or virtual machine.
It highly depends on the version of Windows you want to run. There are differences in this between XP, 7 and 8 for example. The same can be said for OS X (10.5, 10.8 and 10.9 all differ when it comes to use in virtual machines).
The main difference (as you will be concerned) is that the OEM license cannot be transferred to another machine down the road.....so if you buy another mac at some point, and want to re-install your VM, then you will have to buy another copy of windows. This is where the price difference more than catches up to you.
Not quite true. The main difference is indeed that an OEM license is tied to a specific machine and cannot be transferred to another, an ordinary license can be. This is one part where XP, 7 and 8 differ (which is why your explanation is not quite true). There have been Windows OEM versions that were only allowed to be sold with real hardware and thus you can not and are not allowed to run it in a vm (a vm is not real hardware). Windows 8 has seen a big change in this area. The OEM license has become the system builders/full retail license (the naming is a bit vague but it is usually referred to as
PUL). Running it on real hardware isn't mandatory any more. You can use this version for running on bare metal as well as a guest OS (Microsoft even recommends this version for use in a vm).
To make a long story short: read the license agreement of the Windows version you want to run. It'll tell you if running in a vm is allowed or not. The same applies to OS X (running 10.8 in a vm has its limitations!). There is also the possibility that you are not allowed to upgrade the virtualisation software as this changes the hardware too much and thus is seen as a new/different machine (VMware Fusion 6 is known to do this).
Technically you can run an OEM version in a vm without any problems indeed.
To give you an example, take a look at the
Windows XP OEM license for the system builder. Check out points 2, 5a and 5c. The version for Windows 7 is even more strict, it simply tells you it may only be used with real hardware (so no vm use is allowed at all):
System Builder License Requirements. The following clearly explains that in order to use OEM software yourself you have to be a system builder:
Licensing for Hobbyists.
Anyway, for more information about this you can check out
Microsoft OEMs licensing website.