I wonder what the odds are of them changing the look and style of the iPhone when they add 3G. Something tells me that they are going to keep the current look for a while. You can look at the mac lineup right now and not much has changed since Apple did the major redesign on the iMac (I'm not talking about the white plastic to aluminum redesign)
I think there will be an easily visible difference between the two versions. Apple is very PR-aware, and they want the world to know that you have the newest hottest version.
I think maybe that it will get a little longer and a little thiner. The color scheme might not change, but the specific colors on specific parts might.
Anyway I expect that you'll be able to tell pretty easily when someone has a new version rather than an old one.