In all actuality we don't know as of yet. However, from what we are assuming and what we know this will be the new Iphone 5 (or whatever they decide to call it)
-4G/LTE - 4" Screen - Smaller charging/data port - Headphone jack moved to bottom of the phone - 33% Thinner than the Iphone 4s. - Faster Processor (possibly quad-core based on speculation, however to date I am not aware of a Quadcore that can support LTE)
- There are rumors that for the AT&T version the sim card will be even smaller, but we dont' really know at this time.
Press release is scheduled for 09/12/2012, you can read up on what the new Iphone will have then.