Honestly, at first it bothered, but really, it really starts to make sense for me now how it acts. If our hinge would be stronger, it'd be more difficult to open it and might have us actually break it.
Also, when I hold it while typing, I instinctively put my index fingers along the side of the keyboard/screen (once it is opened) and type with my thumbs. That way, it's impossible for the screen to fold back in closed position.
When I pick up my phone, I really don't pick it up by only one of the parts (screen/base). It's actually quite heavy at 180g and if I don't want to drop it, I rather pick up nicely and firmly.
So, really, after 4 days with my phone, the thing that bothers me more now isn't the screen, but the weight difference between my old Samsung Galaxy S (with gel back) and the Z.
Actually, funny anecdote, while playing Angry Birds, I actually opened the keyboard so I had a better grip on the phone.