Got my Desire Z yesterday and to be honest, I don't see a problem here. Its seems to be the design of the unit.
The Hinge.. Mine seems fairly stiff, the screen doesn't rock much when closed / open and besides, it only does when open if i press the top of the screen hard. When closed, its solid. It open and closes with a nice solid snap.
Holding the phone upside down.. I never did this on my Touch Pro 2 and i'm likely not to do it with the Z. While it does drop when I try it, my fingers prevent it from closing.
Holding from the screen.. This is probably the best way to kill your phone. It does drop as described by others but again, I never did it with my Touch Pro 2 and I wont be with the Z. I value the longevity of the hinges and will be handling the phone like any other phone, not by the screen!
I've just bought a nice fabric pouch for it to protect it and keep to parts together safely when out n about.
I'm rather happy with it..
Regards
Craig