Licensing VDI for Microsoft Desktops – is it rocket science?

Given all the past ingenuity and accomplishment why is it, in 2011, the mere task of assigning valid licenses to desktop virtualisation should appear an arcane process?
How do different virtualization models impact how you license your desktop services? What are the current licensing models and do they apply in all instances of desktop virtualisation? Do the models impact on provisioning of services be they laptops, thin clients, Bring Your Own Computer (BYOC), or mobile devices?
Is desktop virtualization licensing an intentionally complex process and what other options could there be?