We already got a big clue about the next iPhone's dimensions from Apples own icon, but Italian site Macitynet claims they've nailed it: an iPhone 4 in an alleged iPhone 5 case shows a huge size difference. Skinny! Flat! Wide!
The gallery shows both an iPod Touch and iPhone 4 swimming with room in the iPhone 5 case, indicating that, yes, the next phone will probably be appreciably wider and thinner. Check Macitynet for the full comparison gallery. Everyone likes thin, but do you want a wider iPhone? Here's another shot, but check out the full gallery at Macitynet.