As long as the ratio of water to antifreeze is correct it won't rust, in fact IMHO its superior than the old method of using Calcium Chloride and water which is highly corrosive. This is still used because it weighs a little more than water/antifreeze and is generally cheaper, but I've seen many rims rotted clear through from small leaks, so you're far better using antifreeze. There are other commercial products out there too that are made for wheel ballest, but probably cost more for the little extra weight you probably won't miss.
The real downside of using radiator antifreeze is that its toxic to animals if there is a major leak, plus its supposed to be sweet tasting and leathal. That bothers me a lot. Personally I load my own tires with windshield washer fluid, cheap and pretty harmless if it leaks. Weighs a little less, but who cares /forums/images/graemlins/laugh.gif