The general consensus is that Apple will be announcing an iphone with 3G capabilities (faster internet, basically) but knowing AT&T, I'm guessing they're probably going to have a new, more expensive data plan to accompany this new phone, since pretty much all of the data plans on their current 3G phones run $50+, though with this boost in bandwidth, they might finally join the 21st century and add MMS messaging.
Some people are guessing that GPS will be included as well. The only other major thing we know we're going to see is the ability to purchase iphone apps on the iTunes store, so I would expect to see Apple pushing a number of new special apps that will add functionality to both new and old iphones. Possibly apps from major partners that they want to highlight.
Given that Google has been pushing their stuff on the iphone, I'd say expect to see something new from them, though most likely theirs will be free and web based. It's possible we may see something that merges current iphone technology with google's web services, such as an app that merges the iphone calendar with the google calendar or something like that.
Some people think that they're going to add video capability to the phone, either for video calling or just to use as a mini camcorder, but this would need a major boost in storage or the above mentioned 3g to make this in any way useful. Personally I think this is unlikely since it's fairly impractical (And who really wants video conferencing on their cell phone?)
I'd say 3g, possibly GPS, something software-based from apple that is pretty but not functionally essential, the promotion of at least 2 or 3 major third party apps (At least one of which will most likely be a game from EA) and something new to integrate with google.
If they were smart, they'd figure out a way to compare people's itunes libraries with the songs that have been approved for ringtones, thus allowing people who have bought CD's to be able to create legitimate ringtones, but I doubt the RIAA will ever let that happen. Also, a cheaper TV show "rental" would be nice, but again, the MPAA would turn that down flat.