If iPhone continues to be the success hit that it is, Apple will like have an answer in 18 months.
The will likely improve form factor, lower weight, reduce packing, increase green materials inside the unit, offer more memory, include GPS features, incorporated a faster CPU (latest generation), improve battery life, change design to include a user-replaceable battery, improve durability from lessons learned from version one, increase camera resolution, include camera flash. Additional (less likely) features may include video capability , some kind of television support, and an integrated FM radio.
I know, I'm speculating...
Somewhere before 18 months, Apple will be working on rolling out the phone in foreign markets, as planned. There will likely be changes to those phones to make them work for those carriers, while otherwise keeping one feature set for all iPhones worldwide.