The big problem that i see with PDAs is that they had their time and now the technology can easily be integrated into a smart phone of some kind. In fact, the phone is a better avenue for it because you get more products in device than with a PDA.
The iPhone doesn't cut it as a phone, or even a smart phone, so if Apple wants to enter into the PDA market, they should just make the iPhone better by adding in the lost functionality of a keyboard, make the thing slightly bigger (Samsung SCH-i760 or LG Voyager) and actually try to make a real smart phone. They would also have to take it off of AT&T's network or extend it to the other, more business friendly networks.
The iPhone doesn't cut it as a phone, or even a smart phone, so if Apple wants to enter into the PDA market, they should just make the iPhone better by adding in the lost functionality of a keyboard, make the thing slightly bigger (Samsung SCH-i760 or LG Voyager) and actually try to make a real smart phone. They would also have to take it off of AT&T's network or extend it to the other, more business friendly networks.