Something I ran into with both Citrix XenDesktop 4 and VMware View 4 and have not been able to find in the standard documentation: when installing a default Windows 7 virtual workstation on vSphere the video drivers default to the WDDM drivers. Both XenDesktop and View however rely on XPDM drivers being available for display acceleration through Thinwire/HDX and PCoIP respectively.
The end result is that with both systems you will get a black screen when you boot up your first virtual desktop session. After about a minute of waiting the session drops. Because it is in neither manual this can be somewhat of a letdown. The fix is easy however and described here (Citrix KB) and here (Jeremy Keen Blog, in more detail). In short: you need to switch to the VMware SVGAII driver which is based on XPDM. See the explanation in the second link for a proper walkthrough on how to change the video driver.
Documenting a virtualization project 