VMware already published a document about the best practices of virtualizing XenApp servers on top of VMware vSphere. You can read about it and download the document over here.
Now Citrix has published a best practice document, the XenApp Planning Guide: Virtualization Best Practics with their point of view on virtualizing XenApp servers. Note that this document not only focuses on virtualizatin XenApp on top of VMware vSphere, but also the hypervisors by Microsoft (Hyper-V) and Citrix (XenServer) are taken into account.
This document also contains a lot of useful recommendations. So I would recommend reading both the VMware document and the Citrix document carefully when designing your virtual environment
Overview of the Citrix document :
Desktop virtualization comprises of many different types of virtual desktops. One option is to use a
Hosted Shared Desktop model, which consists of a published desktop running on a Citrix XenApp
server.
One of the goals when creating a design for Hosted Shared Desktops is to try and maximize
scalability while still providing an adequate use experience. Hosted Shared Desktops provide an
advantage over other desktop virtualization techniques by only requiring the use of a single
operating system, which significantly reduces user resource requirements and helps improve
scalability numbers.
However, in order to get the most users, making correct design decisions as to the resource
allocation is important. Creating too many virtual machines or too few might negatively impact
scalability.
This planning guide provides resource allocation recommendations for users running on a Hosted
Shared Desktop on either Windows Server 2003 or Windows Server 2008.
Note: Even though these best practices are based on the Hosted Shared Desktop model, they are still relevant in a non-desktop model where users only connect to published applications without the desktop interface.
The XenApp Planning Guide: Virtualization Best Practices can be found here.