As the owner of one of many shattered iPods and iPhones, I say 20% stronger is a good start. Maybe plastic is ultimately the way of the future for handheld devices.
As an owner of a shattered iPod and a pristine (but much more aggressively dropped) nexus S, the solution is really curved screens with protective plastic on the edges.
Stronger is good, but resisting edge chips is still a problem for any of these tempered glasses.