I say it needs to be 2000 mAh at the very least, especially since they are not removable and you can't buy a extended battery from another company.
Here are the specs on the previous models:
Original: 3.7 V 1400 mAh
3G: 3.7 V 1150 mAh
3GS: 3.7 V 1219 mAh
It's funny how the battery actually decreased from the original. Why the **** would they do that? Anyways, I think 2000 mAh would be a step in the right direction.