The biggest differences between an iPhone and and iPod Touch are that the iPod Touch lacks an inbuilt mic, doesn't have a camera, doesn't have GPS and it doesn't have cellular network connections. The 1st gen iPod Touch also lacks bluetooth.
From a gaming perspective the differences are pretty negligible: multiplayer over cellular network has huge lag, so it is suitable only for certain types of games and doesn't really hurt the iPod Touch, camera is not used in many games and when it is used, it is more of a gimmick than anything else, GPS could be used in certain situations but again, it is more of a gimmick, and the mic MIGHT be used for voice chat for multiplayer but I don't know how feasible that would be. The mic could also be used for voice commands in games, but that's pretty niche also. So, all in all the differences are not that big when it comes to games.
Looking at the future and the iPhone OS 3.0, it promises some interesting new things. First off is the possibility to play with your buddy via bluetooth: Apple has provided pretty simple APIs to establish a connection and transmit data so I suspect it is going to be a common feature in games. Second major thing are the in-app purchases: think of episodic content, buying new levels, new costumes for characters and so on.
Third new thing is that Apple opened up both the bluetooth and the dock connector for 3rd party accessories for iPhone and iPod Touch, so you could see things like these become a real alternative:
http://arstechnica.com/apple/news/2008/09/rumor-iphone-gamepad-may-get-apples-support.ars
http://www.engadget.com/tag/icontrolpad
Those would solve the control issues, but I do not believe that a great number of people would actually buy them, and in any event the developers would have to design the controls for with and without such a control pad. And that won't fly unless there will be a standard API that all such control pads use.
