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The ability to use 3G would be my first guess ;)

IF you don't have 3G then probably just more storage and less bugs
 
You will be able to receive phone calls while surfing the web, anywhere. (Instead of using EDGE and not getting those calls).

and maybe a new design? Who knows...
 
Way faster internet. Right now what we have can be compared to Dial Up, and the 3G is like Broadband.

3G is scarce in america though, so if you don't live in a 3G area, you will only get edge speeds. I think AT&T will get a move on updating it though asap.

You can receive calls while a web page is loading.

Possibly GPS
 
Streaming video too right?

I think 3G would be a requirement for streaming video to the iPhone. Some of that can be done now, but for decent quality and expiernce, 3G is key.

I think that would open the door to a slingbox iPhone parring.
 
- faster internet speeds
- ability to watch live tv through your handset
- stream video
- video calling
- conferencing

quite a few things to be honest but i cant think right now.

go to http://www.three.co.uk and check their services out. three has the best 3g coverage in the UK with sister netoworks in australia hong kong italy austria and few other countries
 
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