disregarding all other differences, the iphone is a phone, it has a constant cellular data connection, and you can call your friends without worrying about wifi or skype or anything of that nature, I am going to say that that is the biggest difference, just a thought.
And now for something completely different...
I have been reading, and thinking about it and I have to agree with the nays in relation to the camera. As much as I would love to have a touch with a camera, and for me (though I realize that I'm not in the majority) that will probably be the deciding factor between the two. I sorta doubt that they will do that. the big reasons I'm thinking are as follows:
size: the ipod touch is slimmer than the iphone, and I doubt the camera would allow it to stay that way
competition: hardly any mp3 players nor gaming devices have cameras, I doubt apple will feel a need to go in that direction
price: it will add to the cost, lowering apple's money margin
differentiation: it is the most obvious (other than the phone part) dividing factor between the iphone and touch
however, with all of that said, I think that I still might wait until september, just in case, after all I could easily be wrong.