I'll repost what I wrote on Gizmodo:
I guess my first question would be this: while the no-internals iPhone 4 they have might be the real deal, is a real deal development prototyp or production prototype, or is it exactly what you can buy on the 24th? If it's not a production version of the iPhone 4, it might be possible that the glass isn't made to the final spec, since it is more expensive than traditional glass.
Also, it doesn't really matter what kind of glass you use in a phone when you drop it and it impacts on the edge of the glass. First of all, people keep saying, "hey, why doesn't Apple use Gorilla Glass?" FYI, Gorilla Glass IS an aluminosilicate glass and very well could be what Apple is using in the iPhone 4, just without the brand name- you know how Apple is with touting other brand name components they use in their products [www.corning.com]
Compared to one of Corning's other better known products (Pyrex) as well as more typical glass, the mechanical properties aren't too different:
Young's Modulus (tensile modulus-how well it handles tensile and compressive loads, i.e. bending): Pyrex(borosilicate)=65 GPa, Gorilla (aluminosilicate)=73.3 GPa; Soda-lime Glass (stuff bottles and containers are usually made of)=72 GPa
Fracture toughness: Pyrex=0.77, Gorilla=0.7;
Shear Modulus (what comes into play when you drop glass on a corner):Pyrex= 28.2 GPa; Gorilla=30.1 GPa; Soda-lime=29.8 GPa.
As you can see, the strength of all of the glasses that determine how easily it braks it you press on the screen, they're all pretty close but the aluminosilicate Gorilla glass is slighty higher. The shear strength for ALL of the glasses is only about 40% of the Tensile strength, so dropping any of them on a corner will generally result in bad things. The structure of any glass is such that it doesn't handle shear loading on its edges very well, even if it has been chemically been strengthened to add scratch resitance and resistance to failure under bending or impact loads on its top surface.
Gorilla Glass could be susceptible to this kind of damage if dropped on a corner. The only tests I've ever seen of Gorilla glass is your standard rub a key over it's surface or hammer a nail into it and see how it doesn't scratch or catastrophically fail. Never have I seen an impact test from over 3 feet with it repeatedly landing on the corner.
Now, I don't know if what Apple is using is DEFINITELY Gorilla Glass, but it is very, very, likely, it's nearly identical. So, there goes that argument. There isn't any glass right now that Apple could use that wouldn't result in this kind of failure after repeated impacts on the edge of the glass. While it may be more prone to this type of damage since the glass edge is exposed, which is an Apple design choice, I don't think this is at all a case of Apple not living up to the promises it made about it being stronger- it is, just within the limitations of glass. Sure, they could have used plastic or metal, but we already know the issues with those.
I really wish people would do some more research before simply regurgitating one source (that has a bias towards fixing iPhones) or proposing solutions.