I read some articles as to why no new iPhone until September versus June... I thought these were quite interesting (and possible). Of course we'll never know until the new one comes out.
1. It could simply be to put space between the release Verizon iPhone (February) and a new iPhone. (If indeed they wanted to get out an iPhone on Verizon as soon as possible.)
2. Something happened while developing for the next iPhone that caused the delay (overheating of the CPU, perhaps. Needing to fix the antenna issue? Waiting until the competitors reveal their goodies? iOS 5 wasn't ready for prime-time in June; something that is clear, now.)
3. iPods are on the decline, it's possible that they think that September event is no-longer relevant as iPod headlining. Merging the two (iPod & iPhone announcements) is possibly the recipe for a media spectacle.
4. (Most interestingly) To change the release cycle from 1 phone per year, to 2 phones per year (to keep up with competition).
Of course, it could be a combination of some or all of the above. But who knows? Well, other than top-level management at Apple.
As for: does the delay mean major design change? Well the iPhone took 2 years for a major change in design (iPhone 4)... I doubt we'll see something drastically different in one year.
I don't see a problem in changing the size of the screen by .5" or 1" (I'm not a developer), as long as the aspect ratio remains the same, things would be fine, no?