I have to agree with everything in the topicstart. I've been in the same position recently, got a MBA 11" and I thought it would make my old iPad (1) redundant.
But it turns out, I still use them both a lot. I mainly got the MBA to do some development after hours at work (I wait for the traffic to die down, I prefer spending my time doing something useful than to sit in traffic). I got the cheapest-possible 11" refurb (2010) as it's purely for on the road, at home I have a much more powerful mini.
I recently travelled internationally with the MBA and for the actual travel it was horrible. It didn't fit on the narrow train seatback tables (it was wobbling on the thick ridges of the table). It was nearly out of battery after 3 hours of movie watching (20% left or so). I had to use tethering to my iPhone to get internet as opposed to the built-in 3G of my iPad. And on the plane it was too tight to watch at a decent viewing angle when the guy in front of me put his seat back (I couldn't put mine back, being in the back row). All in all much less ideal than the iPad, I hadn't expected it to be that much of a difference.
The iPad is amazing for travelling. I love the way that even when it's off (standby) in my bag it's retrieving my latest emails, that I can watch movies for 10 hours and that there's no screen standing upright (I always use it in landscape mode, slightly tilted in the official iPad 1 case). I do have to to say though, I never attempt to do any serious typing when I'm travelling, just watching/reading/surfing.
However, this time I was going home for 2 weeks and I couldn't do without a computer at my destination so I brought the air, plugged it into a big screen and had a decent computer available there for the duration. Brilliant way to get my personal stuff done and light enough to carry along with my work laptop. The iPad wouldn't have suited at all.
So I agree with what you said about the iPad. It's a brilliant content consumption device but not much good for creation. There's no Xcode, and no good productivity tools that don't frustrate me within a few minutes. I also don't like the onscreen keyboard enough for serious work (and a damn-you-autocorrect situation is not fun in a serious document)
The MacBook Air is just the opposite for me. It's great to get some work done, quick enough to keep up but I really miss the directness of the touch input and the great apps of the iPad, when I want to sit back and just consume. Other than the form factor I don't even see any similarities really 🙂
Next time I'd consider bringing both, but I'm usually stuck with my clunky Dell work laptop and I don't like bringing more than 2 large devices 🙂