I have found a problem with Windows 7 and Windows 2008R2 Guests that doesn't seem to exist with Windows XP guests.
I always park my guests in Unity Mode so that i can use Internet Explorer 7-8-9 from WinXP and Win7 and my databases running in Win2008
if i then Sleep my Host OS, my Win7 and Win2008 will exit Unity Mode and be locked in BlackScreen forever with no way to recover, other than rebooting the guest. My WinXP guests doesnt have that problem, they simply resume and are good to go...
I also experience the problem of BlackScreen if i shut down my monitor (connected with DisplayPort, which causes Windows 7 Monitor detection to engage) -- after turning on the monitor, my Windows 7 and Windows 2008 guest have BlackScreen because Unity Mode failed...
I hope this will help debuggers find and kill the problems with BlackScreen guest