You may have heard one of Apple's claims re. the iPhone 4G that its screen is much more durable and less likely to crack than the previous iPhone. The new iPhone uses aluminosilicate glass on both sides which Apple says has been chemically strengthened to be 20 times stiffer and 30 times harder than plastic.
Sounds great..... Now, here's the result of a 3.5-foot gravitational drop (typical height for the phone to drop if you're standing up) after just three goes :-
Yeah, so I wouldn't be terribly confident, despite the claims. It may well be a much harder material, but that doesn't rule it out from being more brittle and prone to shattering from shock.
Inevitably there'll be a good range of aftermarket cases for it in due course and you'll find something that won't detract too much from the styling. Shame one has to resort to this to keep their beautiful all-glass iPhone from scratches and breakage, but unless you treat your phone like the key to survival, it would be a very wise move.