You know, I asked myself the same question and actually put off buying an iPad for ages reasoning that as I have a laptop and a 32Gb 3GS, I had all the bases covered.
So ahead of another business trip to Spain, I thought "screw it" and just bought one anyway (64Gb WiFi) and you know what, it was utterly fantastic to have it with me. On all 4 flights last week, I had the iPad out on the seat-back tray (using an Incipio Kickstand) and I watched some TV programmes, played some games, read some documentation (Goodreader ftw) and read a book. In the airports with WiFi, I checked my work email, browsed the web and bought a few more apps (I dont want to know how much I've spent on apps this week).
In short, it's brilliant - it does stuff I wouldn't do using my iPhone (the email and browsing experience is so second rate on that and I dont think I could go back to the iphone screen for movies/tv eps now) and it also does stuff I wouldn't do on my laptop (which would be out of juice after 3 hours even with a 9-cell battery).
I actually find the iPhone to be a fairly craptastic phone and (in my opinion), apart from the retina screen, I find the iPhone is now behind the curve in terms of features and so I'm planning to replace my 3GS with an Android device (and use it as a WiFi hotspot for the iPad when I am in a local 3G area) or just a normal (non-smart) phone and use the ipad for everything else.