I was very much surprised by my reaction to the iPad. The larger size really makes for an entirely different experience from the iPhone, to the point where, contrary to my expectations, I perceive no redundancy between the two devices. To me it is a question of portability. I am comfortable carrying my iPhone with me anywhere, my iPad in fewer situations, and my MBP in even fewer. The range of their capabilities, to my perception, are ranged in opposite order.
Maybe it's just the novelty factor, but where I have a choice, as I do at home, my preference seems to be the iPad. Even when I'm doing significant typing, I prefer the iPad with the Bluetooth keyboard to my MBP.
So, I'm looking forward to the iPhone 4, but it won't replace the iPad for accessing the Web if I can have it with me. On the other hand, when I'm traveling light (which increasingly is the optimal way to travel these days), I'm inclined to carry the iPhone and the iPad, and to leave the laptop at home--albeit accessible through the Internet if I need its data.
Now all I have to do is to get over the guilt I feel when I ignore one device too long.