I think the changes are gonna be conservative. Some spec bumps, better camera, and ios 5. I remember the past few years of Apple events, and no device really had a massive leap over its predecessor (with the iphone 4 being the biggest leap, and if i remember correctly, people were still unhappy because of the UI being the same).
I think it'll be ballsy moves by Apple if:
There's a bigger screen
Different form factor from ip4
Announce more than one phone
My full prediction is: Tim Cook appears starts talking about Apple's success for a while. Shows off the new Iphone 5. It look pretty much like the iphone 4 except it has a touch sensitive menu button. Talks about the new specs (A5, more ram, better battery life, etc). He'll emphasize how the graphics are a lot better. Then a popular app developer will appear showing off the new graphics with a new game. Tim Cook reappears, calls an Apple person out to show off the camera and its new features. Cook comes back out, talks about ios 5, introduces another Apple person, who proceeds to give a demo on how the new phone works with ios5. Finally, Cook comes back and tells us the price, and announces that the phone will appear, for the first time ever, on Sprint. There will be a "And one more thing" moment, where Cook announces 3G facetime...and calls up Steve Jobs. (ok maybe that ones a stretch).
The order of how all this happens is probably different, but this is what I'm expecting when i sit down and watch. So if anything more is added I'll be even happier