iPhones lack of some things that the handhelds have:
1. Controls: The touchscreen is good for some things, but not for all things, that's why the DS has traditional buttons, i think if Ninty or Sony go the phone way, that phone will be like some of the Droids or the Nokia N97/N900: Touchscreen with trad. buttons
2. Physical Media: Digital Distro works like a charm with handhelds, but physical media is still favorite among handhelds, but let's say handhelds go full-digital, where they're gonna put the games when the handhelds run out of memory?, erase them and re-download them? using the USB a lot? i don't think so, SD or MS are really convenient, the iPhone doesn't have a physical media jack, the handhelds do have it and will have it...
3. BIG 1st party: Nintendo will always have Mario & Pokemon, those 2 alone move handhelds like crack, now add Nintendogs, Zelda, Kirby, etc. Sony has a lot of 1st party also (moves less handhelds but does the job) and both have big 3rd party support...