I thought I would post my reply seeing as there have been a couple of new introductions into the snow-foaming world.
Both of them offer longer dwelling time - up to 45 minutes, and better cleaning capabilities supposedly.
Autobrite have introduced theirs called Magifoam
http://shop.autobritedirect.co.uk/magif ... 2725-p.asp
and the one that I have just bought Chemical Guys No Touch Snow Foam. The Chemical Guys one is not on their website yet, its only available to Detailing World members at the moment in a trial size. I've not had chance to try mine out yet, but rave reports, reviews and videos led me to try some out, as my current snow foam is getting low, and I need to get some more.
From video evidence that I have seen, leaving it to dwell for 10 minutes at the most, is sufficiently removing enough dirt from the car, that when you come to rinse it off, any remaining dirt is removed. Meaning that the car can almost be dried at this stage, without the need to physically wash the car. As I have previously mentioned, I've not tried mine out yet, so can't speak from experience, but as soon as I do, I'll let you know my thoughts on the new second generation of snow foams.