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My understanding also is that the white is air.
I would expect that the solubility of air in water decreases with the temperature. As the water cools the air comes out of solution ("un-dissolves" ok) making it visible.
The problem for cube makers I guess, is that the freezes from the outside first - trapping the air.
I have heard that cooling the water slower helps. I like Daniels_ suggestion also, to boil the water first. Finally and interestingly - have you noticed that in catering/bars the cubes are usually donut shaped. I always thought this was to increase surface area (and cooling ability). It occurs to me that this will make them look better too.
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