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....

PC's without the PC

For work I have been looking at some solutions for deploying multiple PC's without having the footprint of a full PC on the customers desk. Some choices are Citrix, Blade PC's (where the PC is 1 of many PC blades in the server room) and now VMWare ACE
I like the blade PC idea - I have heard the device that is on the desk is even smaller than a thin client. Only problem (some would say advantage) seems to be there is a one-for-one relationship between number of users to number of blade PC's. This generally adds to hardware and running costs.