Frankly, I'm not sure there's a better thing for most people to say. That's why I suggest not taking it personally.
When I hear "Wow, good for you. I could never do that," I think of it the way I think of "How do you do?": most people who say it are just doing their best to be polite and don't really care that much. In the case of homeschooling, that's probably a good thing, really. But even if they are judging me, so what? As you say, JenSMP, it's a choice based deeply in specifics. They don't know my specifics, so with time I've found it becomes a lot easier to shrug off their misguided judgements.
I guess I think the important thing for non-homeschoolers to know is that there's a period for most new homeschoolers that feels very raw. It's hard to be different, and homeschooling is different. It's a very scary leap to make, and even if you're not being judged, you feel like you are. Often comments from others touch on our own fears of not being good enough. So, yeah, even well-intended comments can hurt.
Maybe since you all know us, you could say something like, "I have some friends who homeschool, and I know it's a big decision. How's it going for you?" or something like that. Something neutral--since I don't know how any of you actually feel about homeschooling and don't want to dictate to you--but that's supportive of the homeschooler's feelings?
Maybe this doesn't work for all cases, but I guess I'd have preferred to hear this over "I could never do that."
The only danger I can see is if you run into someone who doesn't understand that you're just trying to be polite and actually tells you all about how it's going...
