While keeping my fingers crossed, I think I've escaped the hanging grey screen during shutdown since the 10.8.1 update. Startup for me is about 13 seconds from hitting the button to a usable desktop (with a quarter spin of the wheel), while shutdown now feels like one second or less.
I also disabled hibernate on my MacBook -- wake times are pretty much instant for me now, much like Snow Leopard, and the cursor no longer sticks for a few seconds. When I had hibernation enabled, I would wake to an annoying white screen that would stay that way for 3-6 seconds under Mountain Lion.
Does the 10.8.1 update make a difference for you?
Edit: I agree with gnipun's post
on another thread; when applying the update, OS X automatically checked the box to re-open applications, and then cleared the checkbox on subsequent restarts. Perhaps checking the box for one restart, followed by unchecking the box for subsequent restarts, makes the difference?