It is correct that Windows Data Centre on 2008 entitles you to use Hyper-V with as many virtual hosts as you want. Note, though, that this will mean you are now running Server2008 in the enterprise so you will need 2008 cals on all your desktops. It is also difficult to run Hyper-V without MSVCC which is priced per host.
Free VMWare does not provide V-Motion or great things like High Availability. ESX is priced per physical processor.
Don't forget, though, that if, say, you needed 4 physical hosts then that means 8 in Hyper-V (it uses Windows clustering) but only 5 in VMWare as it just needs a spare.
Another option would be Citrix Xenserver which is free and provides Xenmotion (i.e. vMotion on the Xen) but not HA - that comes with Xen essentials which you purchase per host. Again, 4 hosts would only need 1 spare.
Greg