I carried out an assessment on the ROI/TCO for a VDI solution some months ago. Any scenario i worked out, i did not get a positive ROI. here's a gist of how i arrived at the ROI.

ROI = current cost - VDI cost

Current Cost
1. Cost of the Desktop / laptop hardware
2. Cost of spares
3. support costs

VDI Cost
1. Cost of thin client
2. Cost of server (6 - 8 VMs per CPU core)
3. Storage cost
4. Rack cost
5. Network Switch cost
6. Deployment cost
7. Virtualization software license cost

Benefits:
1. Difference in hardware cost (Desktop cost MINUS (server hardware+rack+switch+storage+license+deployment+thin client))
2. Management cost (managing VDIs will be cheaper)
3. Energy savings (Desktop energy consumption MINUS energy consumption of(Server+Thin client+switch+storage)

ROI is always negative as the hardware acquisition costs goes up significantly.

With the ever falling prices of Desktops & laptops, i don't think Desktop virtualization will gain momentum any soon.