New iPhone 5

Ok, the iPhone 5 has changes obviously from the previous model - bigger screen, faster CPU among other things. How far down the line can the iPhone or any other smartphone develop?

There has to be a limit - can't have the screen size too large - then you would be dealing with a tablet, and while I have a Motorola Xoom tablet - I couldn't imagine it also being a phone at that size - of course it is perfectly possible to use a tablet as a phone - would we want something that bulky to pull out in a flash to see who called us? How thin can the phone get? With the untold daily abuse these devices go through with each owner, again there has to be a limit where if it is too thin - how easy would it be to break it in half.

I've been involved in the telecom industry for almost 20 years - watched the mobile phones change from literally a brick to now something that does pretty much everything. What is the next 'must-have' device on these smartphones?