VMware has released special licensing for the vSphere 5 product if you're going to use the Hypervisor for VDI environments.  The licensing is a lot easier to follow.  It's a simple $6,500 per 100 concurrent desktops.  This product is called "vSphere Desktop".  The hypervisor itself will have all the standard functionality of vSphere 5 but you're only supposed to be allowed to run desktop OSes.

The only problem I have is I suppose you'll still have to buy regular vSphere 5 licensing for your infrastructure servers.  For example, if you're running XenDesktop, you'll need to run your Citrix Provisioning Servers, XenDesktop Brokers, & Web Interfaces on normal vSphere5.

I'm guessing this whole thing is going to be an honor system, but I think VMware is making things more complicated than need be.  For more information on this "vSphere Desktop" product & licensing.  Click Here.