Jobs stated (all via Jason Snell): "We knew that if you gripped it in a certain way, the bars were going to go down a bit, like every smartphone. And we didn't think it would be a big problem."
A big enough problem, though, that Apple eliminated Field Test Mode from iOS 4, to make it difficult to find out how big a problem it is.
Sincerely, thanks for the free bumper, though. I'll be using it.