No one really knows what the iPhone 5 will bring but I think there's a 90% chance it will NOT bring a new form factor and/or major changes. My thinking:
Major Changes, 10% Chance
Apple must keep up with Google on the hardware features front even if iOS is arguably more refined, stable, intuitive and easier to use than Android. The OS alone being superior won't succeed in the long run, Android is getting more and more refined by the month.
No Major Changes, 90% Chance
One word, "Verizon". If indeed we see a Verizon iPhone announced mid to late January 2011 it'll most likely be an iPhone 4 of the same specs as the AT&T iPhone 4. Can you imagine Verizon getting the iPhone 5 before AT&T? I can't. On the other hand can you imagine the opposite? Verizon getting the current iPhone 4 and then 3-4 months later AT&T getting the iPhone 5? I believe Apple has got to bring the two carriers in synch from the get go so I'm leaning towards "No Major Changes". Perhaps just a speed and/or RAM bump for both carriers as this would not take that much engineering effort.
Of course Apple could announce the iPhone 5 mid to late January 2011 on both carriers but not be shipping them till June 2011, but I doubt that'll happen. Apple typically hasn't done this except for when the first iPhone was announced.