Here’s a CTX article I found while troubleshooting gray screen issues with XenApp target devices. I implemented the Microsoft hotfixes and the issue seems to be resolved.

http://support.citrix.com/article/CTX125761

A XenApp Target Device appears to have locked up or hung. The Provisioned XenApp server is accessed through the local console; you see a gray blank screen. There has been different symptoms reported for the same incident and they are not specific to Provisioning services or XenApp. In some cases the console appears to be frozen at the screensaver for that Target Device. Although the Target Devices appear to have hung, only certain services on the server have stopped responding. A reboot normally brings the Target Device back to service.