Q:

Make an iPhone appear as different model? 

Is it possible to have a jailbroken iPhone trick an app into thinking you have a different device than the one you're on (e.g., a 3G passing as a 3GS)? 

A:

Palm did this with the Pre: the device, like all devices, told iTunes it was an iPod and allowed only the syncing of non-DRM content over the USB connection.  As to how the iPhone sorts out what it is to tell iTunes could either be hard-coded or come from a plist file somewhere in the OS.  
Either way, this forum is really not the place the find the answer to that question(nor do I think SO is either).  You probably should head over to one of the myriad iPhone jailbreak forum.  Searching "iPhone jailbreak forum" in Google brings up a whole host of them.