I wonder if that's actually a good idea -- I mean, from a green perspective, the best insulation is "air", so most insulations (fiberglass, double-paned windows, etc.) are all "tricks" to trap air. Bottles are already trapping air, so it's just a matter of arranging them in a pattern where they reinforce each others' weak spots (e.g., pattern them alternating up and down, with the neck of one between two bodies of another, then create a second layer to reinforce the weak spot, which would basically be doing the same thing at a second layer).
(wanders off to think about it further)