Depends on what you use them for...
I LOVE my Macbook Air 11". I also love my ipad. Some people think the MBP 11 is too small to do actual work on but I don't have a problem with that (I'm a designer). The size and weight difference between that and the 13 really mattered to me as a road warrior. I can always hook it up to the TV or to my iPad as a second monitor, with AirSharing. I would probably get a MBP 15 if I needed more screen size and power.
TRAVEL COMPARISON (ipad 3 vs. MB Air 11)
My Macbook Air 11" is about the same weight as my iPad 3 with a keyboard, so that's a tie. The power cord of the ipad is less bulky so 1 point for iPad.
UPDATE: Apple just announced the new iPad Mini. I could see getting that, along with a fold-out or roll-up keyboard, if I really wanted to go ultraportable.
iPads are great because you can carry it standalone, it has the touchscreen, it's less intrusive if you're talking to people or collaborating. It's not so great if you want to do real work. If you're just writing, emailing and casual web browsing, iPad is fine. But I've had to buy apps to do "just a little more" with the iPad, things I could easily do on my MBAir (PDF, FTP etc). And I can't roundtrip my Illustrator flowcharts or Photoshop web designs on iPad at all.
There are many things the iPad can't do. That's because it was not designed as a "real" computer. It's a mobile device.
Another thing to consider is access - where accessing secure sites and apps on your main computer is second nature, it can be a pain on iPad. I use 1Password on both devices to make life on the road much easier.
Lots of neat keyboard cases out there. There are some cool kickstarter projects - one that offers a leather case to carry both ipad and the apple keyboard (touchtype, not currently taking new orders) and another that basically turns your ipad into a laptop (crux skunk - cool features, available but a bit pricey). I prefer using the Apple wireless keyboard because I don't have to learn idiosyncracies of other keyboards, and then get tripped up when I switch back and forth between devices.
I am still searching for the ideal case to marry the two. Meanwhile I use the keyboard socket from SFBags.