Note that the greyscreen occurs even with just one VM running, but I've found that it's more impressive when multiple VMs all go grey at once.
To replicate the issue, the VM's "Options-->Full Screen" configuration should be:
Use Mac OS X Full Screen ENABLED
Active Screen Corners DISABLED
Scale to fit screen: Auto
Use all displays in full screen DISABLED
Start the VM, set it to Full Screen mode, and use Mission Control to drag the VM's screen to the secondary display. On my system, the VM screen will eventually go completely grey.
If multiple VMs are running, each in their own fullscreen on the secondary display, they will all go completely grey at the same time. This indicates to me that the problem is not with any particular VM, but with the Parallels Desktop video subsystem.
If a VM is not in fullscreen, or has the "Use all displays in full screen" enabled, or "Use Mac OS X Full Screen" disabled, the greyscreen does not occur.
I launched four different VMs, and generated a problem report for each one that went grey simultaneously (interestingly, the Win7 VM did not go grey with the rest of them this time as it usually does; no idea why):
1. Windows XP (report ID 45262351)
2. Linux Mint 15 (report ID 45262423)
3. tails 1.0.1, without Parallels Tools (report ID 45262474)
