I worked some mock-ups in my cad program and a 4" screen will fit: Of interest is, they made the iPhone 4 narrower than the iPhone 3 (they took away the bezel and shrunk the case width closer to the 3.5 screen and squared it off). SO, if they widen the iPhone 4 case back out to the iPhone 3 width, keep it squared off, theres room for a 4 screen (and keeps the same aspect ratio). So I'm betting the iPhone 5 (when ever that happens) will have 4" (yeah, sad this is how I spend my lunch time) See attachment: A quick update to add a few points. Apple squared off the iPhone 4 and as a result had to make it narrower so it would still fit in the countless iPhone docks on the market (see image below). Keeping with this new limitation, we can figure that a 3.8" screen is all we'll ever see in this form factor.