Have you considered a Super Zoom, the Canon S3 IS comes to mind. Roughly half the size of a DSLR, I have seen some amazing work done with that little beast. My wife has the S2 IS and I have been very impressed with some of her photos. Outstanding movies too, I've read.
The other suggestion I would throw out would be to add the Canon 28-135 IS to your battery of lenses. Cartier-Bresson seldom if ever worked out of that range and he seemed to do all right. I have that lens and it was my only lens for my D60 until earlier this year when I went a bit nuts with a new 30D. As long as you have the 5D, I'd look into that one. It's a very nice lens. Right now, I'm running the 17-85 IS on my 30D (the same range in 33mm equiv) and use it >90% of the time.
I also have the G7 and it is cool! Still a bit to learn to maximize it, but I think I'm falling in love.
Edit: I've gone back and reread the original message and offer this, the 70-300 IS, not the DO version, is also a mighty fine piece of equipment. Mine is very sharp, you can watch the IS kick in, and most importantly, not much larger than the other Canon lenses I've mentioned. Mine was $679 but I'll bet they could be had cheaper. The DO version is what, $500 more? I think this lens would be great from an airplane, just watch for scratched windows, but you probably do that anyway.
Actually, I think you need one of the 8mp 28-200's Michael reviewed some time ago, mine is the KM A2, unfortunately hard to duplicate today.
Bill in WV