I'm actually thinking of going back the other direction.
I had a 17" MBP + iPad that I used as my travel combination. Worked well...on long, overseas flights I could use the iPad for movies and some games, and I've always been a big fan of having the highest resolution screen I could get.
But it was a huge pain going through airport security, and my shoulder isn't what it was 20 years ago.
So the last time I was in the US, I picked up a loaded MBA 11", thinking it could replace the iPad, AND handle some of my workload.
Unfortunately, I found the 11" MBA (2010 model) a little underpowered, and just a tad small for my tastes. So I swapped it for a 13" MBA fully loaded, and have been using that as my primary machine ever since.
Work also provides me with a 15" MBP. That machine has mostly sat at my desk at home doing very little. Because it's a work machine, I'm hesitant to load up my iTunes music, and other media, games etc on to it. Not a fear issue...just a preference. I'd rather not become dependent upon a machine that I don't directly own (work could decide, for example, that they won't supply Macs, and will only supply Windows PCs going forward).
However, on the road, I've found that the 13" MBA is still somewhat underpowered for what I'd like. I prefer the extra HP of the Core i5 in my 2010 MBP (work), and the dedicated graphics chip (occasionally I'll play some games back in the hotel room).
I also find I prefer the glossy display on the MBP, and this is going to sound silly, but typing and using the touchpad on the 15" MBP just feels more 'solid' than on the MBA.
So I'm actually leaning towards going back the other way....picking up a loaded, 15" MBP (high-res glossy display version), and an iPad2 as my travel companions.
To me, the greater capabilities (storage, connectivity, etc) outweigh the greater weight of the 15" MBP vs the 13" MBA.
If the new 13" Core i5 MBA had come with an 8 GB RAM option....that might sway me though. Too bad.
Patrick