I sold my iPad 2 for an 11" MBA. I also have an iMac and an iPhone 4S.
I use Dropbox to sync my important files (mainly school ones), in which I can then easily access everything from any of my three devices. All three devices have the same music; photos/videos are all on my iMac.
Whenever I go anywhere, I of course have my phone. When I go to class, I always throw my MBA in my backpack, as it barely adds any weight or takes up any space. My iMac obviously just sits on my desk and it's where I do most of my work. I'll typically only use my MBA when I'm laying in bed or in class (though my girlfriend uses it a lot when she comes over).
So yup, I wouldn't have an MBA and iPad at the same time. For what I use them for, the MBA comes out on top. The only thing I miss is the iPad's battery life =|
I'm still excited to see what the iPad 3 will be like though.
This is the exact configuration I have right now (minus the 4S, I have the 4). However, as an Arts student who does a lot of writing, I have found that my iMac is essentially irrelevant. Yes, I enjoy sitting down at a desktop far more than I enjoy hovering over the 11" display. But, my iTunes library isn't extensive, so my iMac is not needed as an entertainment hub. My school file list isn't extensive enough to overflow the 64GB of onboard storage on the Air (in which case I will just use online storage for files). And with the SSD, I find my Air is just as fast, if not faster, than my iMac at doing the primitive tasks that I am consistently asking it to do.
Long story short, I have decided to sell the iMac and grab a Thunderbolt Display. This way, I don't have the problem of syncing files and always wondering if I transferred the files over properly. I have one computer with the "iMac-like" experience when at home.
And I can proudly say my main machine is a 64GB MacBook Air...