With a mere two weeks before launch, Apple has announced two last-minute changes to the iPhone's design that will hopefully (for them) make it an easier sale and hopefully make it a better product. Changed from the demo units they have showed off in the past is the battery, which now will last for eight hours of talk time, and the cover, which has been changed from plastic to glass.
A glass cover will make the display clearer and more scratch-resistant, a definite plus and a lesson Apple has likely learned in great depth from the iPod Nano's release. However, if you're going to be sinking $600 into a phone, you'll probably want to buy a screen cover to protect it anyway.
A higher-capacity battery is a definite plus all around, especially with a device that has so many nifty yet battery-draining features. In the iPhone it is particularly critical, since the battery isn't user-replaceable. Obviously, changes this late in the game must have already been made pre-production, so Apple is just looking to get more people interested in the device.