I think you're spot-on considering how cheap NG is, and payback times getting longer and longer. I'd think though that given the huge amount of solar equipment on the used market -- and the way you see more do-it-yourselfers in a down economy anyhow -- there are going to be a lot of people looking at super-simple setups, done themselves, and on the cheap.
I've sort of become one of those by accident; I couldn't pass up a pair of 20-year-old collectors that didn't leak for $50, so I'm probably going to do a pre-heat DHW tank just as you describe, with glycol and a heat exchanger. "Free" BTUs are hard to pass up.
But new equipment, pro install... it's going to be a tough sell indeed. Probably the growth market on all this is going to be post-DIY service calls.