Unfortunately, I have the same problem. The bottom of my screen lost sensitivity after only about 20 days of use. First the very bottom of the screen went out so I could not hit the space-bar on the keyboard, or change to the number/character screen. Then it got worse a few days later, now I cannot hit any of the main function buttons. I have no Phone, no Safari, no iPod, and no Mail... I can barely get the slider to work when unlocking.
I have gone through the reseting procedure as well as restoring it completely, This made no difference as it is clearly a hardware issue.
It's too bad that there are so few physical buttons because when the screen goes out, it is seriously useless.
Apple should not be charging people for a temporary phone while repairing this type of thing, it is an obvious manufacturing issue, and it is costing my business money for me to be without my phone.
I have yet to get this repaired, as I am in China on business, so I'm totally screwed. I have to go out and pay full price for an antiquated replacement phone out here.
I do really love this phone, it is by far the best consumer electronic device I've ever used, but that makes me even more bitter that I'm stuck out on a business trip without it, and forced to use a cell phone from another company stuck in the ice-age.