Before buying the new iPad (my first), I also owned a MBP and a smartphone (currently a 4.5" Samsung Infuse android phone).
Generally, I'd do what I wanted to do on my phone while out and about, and at home, I'd use the MBP.
"use", for me, means:
* browse websites
* read RSS feeds
* check various email accounts (work and personal)
* participate in various fora
* facebook
* online banking
* track my training
* calendaring
* shop
* watch videos and movies
* listen to music and podcasts
* read
* various programming tasks (both recreational and for work, sometimes via VPN)
All but the last I could do on either the MBP or my phone, to a greater or lesser extent.
After buying the iPad -- and bear in mind, I was one of the people who derided the original iPad as a superfluous, oversized Touch or iPhone when it came out (I had an iPhone at the time, and will be returning to it shortly) -- I found that I no longer need to use my MBP at home. Which is nice, because it means I don't have to lug the thing around from living room to bedroom to downstairs office. I can leave it in the office, next to my desktop, hooked to the external storage I keep down there. I can use the iPad everywhere else, and take it with me when I travel (I've long since stopped taking my MBP on trips).
With an Apple bluetooth keyboard and one of a few different apps, I can even ssh into various machines at work via the iPad and be productive, if I need to.
And sharing media with my partner while we're relaxing is much easier...it's trivial to flip the iPad around and show her something, while rotating the MBP and putting it somewhere she can see is a bit more of a pain.
So, it's pretty much replaced my MBP, and largely eliminated the way in which I've been using my smartphone in all but certain circumstances (e.g., while in the car).
