Historically, the benefits of virtualization solutions have been offset by a significant performance overhead, with many fewer users being supported on a virtualized server than on the native (“bare-metal”) system. However, with Citrix XenServer - and indeed the latest release of VMWare's ESX server, scalability concerns are being out-weighed by the benefits delivered by this virtualization solution.

HP  and Citrix have released an interesting document - the snappily titled Using Citrix EdgeSight for Load Testing to characterize HP ProLiant server scalability in a Citrix XenApp on XenServer consolidated environment  uses Citrix's excellent Edgesight for LoadTesting tool (which is now a free component of XenApp) to compare load versions across bare metal and virtualised servers. While some of the low end server specs appear a little light in terms of user load (from my experience) - there's definitely a case for re-visiting virtualising Citrix/TS servers whic, in a year or so ago, typically generally resulted in  poor performance and user experience.

This is a useful addition to the growing amount of information in this field. If you are investigating this area, be sure to check out the excellent work by the bods at Login Consultants with their free VSI 2.0 tool which can be used to benchmark SBC and VDI environments - and Project Virtual Reality Check - which is a joint venture of Log•in Consultants and PQR, who have researched the optimal configuration for the different available hypervisors.