The foremost of these changes is a new, front-mounted camera, which will make DSi porting much easier -- in other words, "put your face in the game" gimmickry could become common in new iPhone games. Also, according to Gizmodo's preliminary findings, the new hardware's screen appears to have a higher resolution than the 3GS screen (at least, it's smaller in size). The back of the prototype does not reveal the amount of internal memory in the device, but previous reports have suggested the next iPhone will have up to 80GB of space (which would be a stupid silly amount of games). Additionally, there's a larger battery installed in the lost-n-found unit than the one in the 3GS.
Finally, the prototype sports a new shell with a flat back, which apparently feels "freaking amazing." That's always a good thing for handheld gaming, right?