Of course Luna meant for 7800 run fine on my 6600 GT too, although it's my least favourite demo so far.
Luna ran on your 6600GT because that gpu supports all the features used in Luna. As far as FEATURES goes, the 6 and 7 series really aren't all that different. All the 7 series has that the 6 series doesn't is some add AA methods and I believe full hardware support for PureVideo. Other than that they have the same featureset. So if your 6600GT didn't run luna then I'd say there was something wrong with your card.
As for the 5700 running nalu, I'm surprised. Maybe it reverted to SM2.0 for your card so it'd work? Cause the NV3x series does NOT support hardware SM3.0 at all. Hm, who knows.