Don't underestimate IT departments. We buy new Windows 7 machines, wipe them, and put XP on with our volume license. I really hope that the pending end of security updates forces us on to Win7, but I think we'll keep using XP until hardware vendors stop making drivers for it.
Once Win8 is released, new machines will be sold with Win8, and then available for downgrade according to Win8 downgrade rights.
I haven't seen details of the Win8 downgrade rights yet, but previously they allowed downgrade to previous 2 versions... i.e. Win7 downgrade rights currently entitle use of XP.
It will be interesting to see if this is rolled forward as downgrade rights to Vista, and hence eliminating downgrade rights to XP.
Of course, how much attention anyone pays to such a limit (if even imposed) would be another matter.