VMWare Virtual Desktop Infrastructure - Many PC's on one Server

Although I haven't fully researched any of these solutions, VMWare VDI sounds exciting. It runs on VMWare ESX server and allows anyone with a thin client (I have also heard it is possible with a web browser) to connect (even at boot-up without the need for Remote Desktop or VNC etc) to the VMWare server. The end user experience is exactly the same as if they had a PC at their desk. Also there are snapshot modes (to enable roll-back of patches/software installs) and the option to refresh the user environment to the way it was (when it was created) whenever the Virtual Machine is power cycled.
What I need to figure out is how beefy a server would have to be and how many servers would be needed to handle 100's of PC's at one time....