There are a few improvements that seem like no-brainers. With each iPhone, Apple has made the built-in camera better. The iPhone 4's rear camera is great; I have no doubt that the iPhone 5's cameras--both of them--will be greater still. Similarly, each iPhone is faster than the one that comes before it, and it's thus an easy bet that the iPhone 5 will feature some speedier internals.

Beyond those two improvements, I don't think there are any definites on the hardware side. At least, not until another iPhone prototype shows up in a bar in the next few days.

: It's possible the iPhone will get bigger. It's possible that the form factor will remain the same, but the screen will take up more of the iPhone's surface area--extending all the way to the edges, or inching closer to the ear speaker. I must believe that if Apple does change the screen's dimensions, it will do so in a way that doesn't require that developers rejigger their apps' artwork--something they already had to do first with the release of the iPad and later with the advent of the Retina display. Asking developers to create yet another set of artwork for a slightly larger iPhone display strikes me as more trouble than it's worth, so Apple will likely keep the total pixel count the same.