I'm the network admin for a school. We're currently happily running 70-100 VDIs (depending on the time of day), and will be going full scale up to 600 next summer.
For us, storage was the biggest pain. We tried running our trial VDI's alongside our servers, and quickly ran out of IO. We splashed some cash on two new SANs with a TB of Cache and Dedupe, and are happily outperforming our fat clients. Login times are now ~ 1 minute as opposed to 2 mins+.
For VDI get your numbers right. There's a good tool called Quest VDI Assessment. Run that on as many desktops as you can, and it will tell you your average and peak disk IO, memory and CPU. After that it's just a case of doing your sums, planning for some failover and buying some new toys.
We're now looking at Samsung PCoIP thin clients. So far very good, and have just watched the Matrix full screen streamed from one of our virtual servers.