My problem with this idea is a couplefold - I also hike in places where the water is cold.

First is I find my water warms up better in a platy that is against my back, and the warmth from me is a bit faster than trying to warm up water lashed awkwardly on the outside of my pack in the sun.

The second is that if the point of carrying less fuel, I'm thinking I'll cary the extra 1/8 oz of fuel per boil to heat up the cold water I boil, than try to walk with an extra two pounds of water while it warms up.

Now it's a nice idea if you're lazing around in camp, and have time to leave the container out in the sun. but of course you can't really count on the sun being there either.