I'm on my 3rd iPhone. The first one, I had for about 8 weeks. I exchanged it because it had those yellow spots and they wouldn't go away. During that time, I never kept it in a case. I dropped it once on concrete, ended up with a couple of scratches on the metal, but otherwise it was fine, no marks on the glass on either side. I also dropped it couple of times onto hardwood floors, survived fine both times.
The 2nd one I had for 2 days. I returned it almost immediately when I found it had the proximity sensor issue. Didn't drop it during the 2 days.
My current one, I've had for over 2 weeks, again, no case whatsoever. Sadly I haven't dropped it yet. I do have a few light scratches on the metal from being in my pocket with coins and keys. I really should stop doing that. As far as I can tell, there are no marks on the glass yet, but it seems I've triggered the water sensor on the dock connector. Might have happened when I spilled some coffee on my desk. I wiped the phone off and used some Windex to clean it up.
Overall, I would say this iPhone is no less or more sturdy than previous ones. I presume the back glass is a lot more fragile than the plastic on previous iPhones, but the front glass is supposedly much tougher.