In the most recent iPhone rumor via iLounge it appears that Apple is going to be producing two different iPhone models differentiated by data speed.
The "slow" model will be a 3g phone. The fast model will be an enhanced 3g model supporting 3.5/3.75g speeds. Americans on AT&T will most likely get the enhanced 3g model to support AT&T's upcoming 7.2 Mbps data network. Customers will not get a choice on whether they want the normal 3g or the enhanced version. Versions will be sold based on on country and carrier capabilities. The third model of the phone to be thrown in the mix will be a Chinese version for sale exclusively in China.
While you wont be able to pick which speed your carrier is offering, you will be able to pick storage size. It is still up for debate whether it is going to be an 8/32GB choice or a 16/32GB choice. We don't think Apple will be offering any 64GB models.
Finally iLounge also corroborated the rumor that the next iPhone will have a rubber back instead of the cracky plastic. What a shame -- the metal back was my favorite.