The license for Windows Vista has gone under yet another revision, this time to be more compatible with virtualization and thin-client computing, two increasingly important topics that will become crucial over the next few years. They've added two new separate license options through Software Assurance, one each for virtual environments and thin-client environments. The license for the “Centralized Desktop” edition will be subscription based, interestingly enough:

The subscription-based Windows Vista Enterprise Centralized Desktop (VECD) license lets users run Windows in virtual machines on a central server, while a second license lets users run Windows Vista on diskless PCs with storage managed centrally and the client using shared images of the operating system.
These changes are actually good, and make Vista easier to work with in business environments that may want to deploy thin clients.