Citrix has released a good whitepaper on how to delivering 5000 Desktops with Citrix XenDesktop.
The validation report and recommendations includes how to obtain a Scalable VDI Deployment using Citrix XenDesktop and Provisioning Services, NetApp Storage and VMWare Server Virtualization.

The document is intended to provide advanced technical personnel – architects, engineers and consultants – with data to assist in the planning, design and deployment of a Citrix XenDesktop hosted VM-based (VDI) solution that scales to 5000 desktops. This document presents the findings of internal Citrix testing that simulates a large enterprise deployment of VDI desktops.