I have successfully replaced my laptop with my iPad 2, as all I ever did with it was internet and e mail anyway. I don't even have a laptop anymore. I carry a backpack pretty much all the time, and my iPad is ALWAYS in it. It's light, easy to use on the train, and suits my needs perfectly.
I have found more and more that I have completely different apps on my iPad than I do on my iPhone. I'd rather surf or search for things on the web on my iPad. Same goes for eBooks, magazines, writing up documents, FaceTime/Skype, e mail, photo projects, Netflix/Hulu Plus, games, etc. At the end of the day, my son always insists on reading books on the iPad before bed. (BTW, The Fantastic Flying Books of Mr. Morris Lessmore is an AMAZING kids book.)
However, since getting an iPad 2, I also feel that it has redefined and in some ways rekindled my love for my iPhone 4 as well. An iPhone 4 is a computer in your pocket, and it's all I need when I want to travel light. I prefer NOT to read books on it anymore, but I will if I'm bored enough. Some gaming lends itself better to the iPhone than the iPad. I prefer Safari on the iPad, but in a pinch, it's great to be able to pull out my iPhone and do a quick Google search. And the obvious use--listening to music--I don't think I've done this once on my iPad, even though I do have music on it. I think 11 of my 16 GB is music.
But in an ideal situation, I have BOTH devices with me.