In February, Citrix published the results of an internal benchmark about virtual machines density of a single XenServer 5.5 physical server part of a XenDesktop 4.0 environment.
The measurement was performed using the independent workload simulator Virtual Session Indexer (VSI) and Citrix reported up to 130 concurrent Windows XP virtual desktops served by a a 72GB, dual socket, quad-core Intel Xeon x5570 (codename Nehalem) host.
A few months later VMware published its own result after running the same simulator with a slightly different configuration on a similar hardware (so no identical scenario) hosting ESX in a View 4.01 environment.
The machine with more RAM and same CPU scored 170 concurrent virtual desktops, while the machine with less RAM and less powerful CPU scored 142 concurrent virtual desktops:
