It's not been announced, so the simple answer is "no-one knows". However, if you take into account the fact that most of the iPhone is software driven and based on OSX, then there's a pretty good chance that most of the new features that find their way into iPhone 2 will also work with current iPhones.
The only things that will be unique on iPhone 2 will be hardware based and I would guess at the following as possibles:
3G (a given)
Larger capacity (almost certain)
Forward facing camera (maybe)
Er, that's about it