There's going to come a point where thin = sacrifice. So far, we've been spoiled by good designers that figure out the math to give us what we're used to in a thinner and thinner package...
I believe that time is quickly coming to an end. We, as consumers, demand more battery life, more power, better cameras, better features and new technology...and we demand it no more than a minute or so after the latest and greatest is sold to us.
Bottom line, until some major break throughs in technology, battery tech and screen development come along, what we have is about as thin as it's going to get. And honestly, I don't think we're going to see these major tech breakthroughs on an iPhone this year anyways.