A couple of weeks ago Citrix published a new architecture blueprint for its VDI platform XenDesktop.
The 38-pages document provides guidance to design scalable virtual desktop infrastructures based on Microsoft Windows Server 2008 and SQL Server 2005, Citrix Provisioning Server 5.1 and of course XenDesktop 4.0 (which includes the hypervisor and XenApp 5).
In this paper Citrix doesn’t push for the adoption of XenServer 5.5 but highlights that XenDesktop is hypervisor agnostic and can work with VMware vSphere and Microsoft Hyper-V as well.
The company doesn’t even detail if and how different hypervisors will impact the scalability of this architecture but it offers some reference metrics in case you plan to use XenServer.