It doesn't take a genius to see why multisite VDI might sound like a really bad idea--VDI can be a bear on the WAN. Getting exact numbers is going to be tricky, but Daniel Feller, who runs the Virtualfeller blog, does a pretty good job explaining what it's like to run Xen Desktop across the WAN. He estimates, for example, that you'll need up to 85Kbits per second for Internet browsing per user and 553 to 593 Kbits per second for remote printing per user. Multiply those numbers by the duration of activity, and an estimated traffic profile can be obtained for each user. Keep in mind that those numbers can drop significantly based on a number of factors, including whether you can leverage Xen's Branch Repeater.
Lest you think Xen is alone, though, take a look at VMWare's expectations for VDI requirements. A good rule of thumb when running PCoIP is three users per 1Mb. This allows for variance in the display activity between multiple users and provides a range of bandwidth most likely to provide acceptable performance for user.
While deduplication technologies provided in WAN accelerators can address some of the bandwidth issues, it's not just bandwidth that matters here. Out-of-order packets cause retransmissions higher up in the stack that decrease application performance. Addressing packet ordering is particularly important in an application like VDI, where responsiveness will be very noticeable.
How packet ordering is treated can make a significant difference in the performance of VDI. This is a big area for folks at Silver Peak who use Forward Error Correction (FEC) and Packet Order Correction (POC) technologies to address the problem. FEC is used to rebuild dropped packets on the far end of a WAN link; POC is used to re-sequence packets that are delivered out of order.