In terms of design, I've always been a fan of the 4 and 4S, because they look lovely. It was a nice evolution of the first three iPhones. However, the following design has been taken by most websites as what the iPhone will look like:

I actually really do like this, as it is again an evolution of the 4 and 4S, but it isn't drastically different. There are also rumours that the new iPhone will be 16:9, as opposed to the current 4:3. This gives quite a bit of extra space, and would go with the screen going up to 4 or 4.3 inches, and would at least mean that the iPhone has caught up with the rest of the pack.
There's also supposed to be a new dock connector, with less pins, and new headphones. The actual phone itself is iOS6, which doesn't really have any stand-out new features, unlike iOS 5 with Siri.
Whatever Apple come up with tonight, it'll certainly be very interesting.
I'd love to hear your thoughts on it all.