I'm on record as not being a huge fan of the iPhone as a phone. Ergonomics aside, I'm deeply grateful to all those TUAW readers who pointed me to a bluetooth earpiece solution. Sure I look like a dork with that thing in my ear (not much difference, mind you, from my normal look) but with my 8-dollar unit (thank you Tuesday Morning clearance aisle), I can talk pretty comfortably for long periods of time without holding the iPhone to my cheek.
When chatting, that iPhone normally stays either in my pocket or in the front holder in my stroller while in use. With the iPad, it would be pretty darn hard getting it to fit in either location. If I were to use the iPad as my main cell phone, it would have to be dragged along in a backpack no matter where I went. And, frankly, I like grabbing my keys, my wallet, and my phone and heading out the door without a backpack, a purse, or a man purse satchel.
Let me give you another real life example of where the iPhone outshines the iPad. I love tracking my walks using TrailGuru. There's no way, I'd do the same with either a netbook or an iPad. It's just the wrong solution for that kind of need. They're simply too big. Fact.