Recently I was asked about the performance and scalability of XenApp from our CSA. And I got the following reports or informaitons. It will be helpful to everyone to check these reports and informations at single site.

XenApp 5.0
XenApp 5 Scalability Analysis

Citrix Presentation Server 4.5
Scalability – Number of Users per Presentation Server

Effects of Varying the Number of CPUs of a Citrix Presentation Server

Citrix Presentation Server 4.0
Citrix Presentation Server for Windows Server 2003 x64 Editions – Performance and Scaling Capability

MetaFrame XP
How Hyper-Threading Affects User Capacity of Metaframe XP Servers

Advanced Concepts Guide for MetaFrame XP with Feature Release 3 – P79
Effects of Varying the Number of CPUs and Hyper-Threading on MetaFrame XP Servers

MetaFrame 1.8
Windows 2000 Terminal Service Capacity and Scaling

Other sources

Test tools
Citrix Server Test Kit Users Guide 2.1

EdgeSight for Load Testing 3.0