Try browsing the web or checking email for any length of time on an iPhone. Not very good, huh? But fine on an iPad's big, comfortable screen. Or try using a notebook for 9 hours. Try holding a netbook or a notebook for even one hour. Gets pretty heavy after a while, doesn't it? Not an iPad. Try reading an ebook on your patio with a notebook. Doesn't work so well, does it? An iPad is like a Kindle, very comfortable and natural.
An iPad won't replace an iPhone (really apples and oranges), but it can replace a heavy, short battery life notebook, especially in the areas of browsing and email.
I love mail and safari on the iphone because it is so much easier to type on the smaller keyboard. I can type just as fast on an iphone as a physical computer keyboard. Using an ipad for 9 hours??? My arm got tired from holding it after 10 minutes! However, I do like the the bigger screen for books, you got me there, but that to me can't possibly justify the cost. If it was 100 dollars... MAYBE. The e-reader thing is the only thing about it that appeals to me so for me to be interested it would have to be priced competitively. But I just used my girlfriend's stepdad's new iphone 4, and let me say it is just fine for book reading. Why would I hold a notebook???? They are made for either a lap or a table, so one should never have to hold one. However, one pretty much has to hold or somehow prop up an ipad... it was probably one of the most uncomfortable things I Have ever used. My MBP will last all day as far as battery goes, but my point is that with either an ipad or a notebook you have to carry it in some sort of bag, but an iphone can fit in your pocket and it can do what an ipad can, some things better because it is easier to control a small screen with the human hand.
And I would call it a "big comfortable screen" for viewing purposes only (though the iphones is plenty big for everything). It is so much harder to type and just do things on it than an iphone because the size of it is cumbersome to the human hand.