I've seen a glass disk placed in a pot and allowed to rest on the bottom used to prevent boil overs. In search for the name of this thing I found this at wisegeek;
"Boil over tends to occur whenever a ingredient added to hot water sheds a starchy coating. This coating floats on the surface of the water and forms a foamy layer over time. The foam, most notably from rice or pasta, tends to prevent natural ventilation, which makes the liquid become superheated. When the liquid becomes too hot and the foam is not skimmed off or stirred in time, a boil over can occur. In theory, an effective boil over preventer would absorb this extra heat and keep the temperature below or at the "
They also had this to say about the glass disk;
"A common boil over preventer found in many stores looks like a glass or ceramic disk. This type of boil over preventer is placed on the bottom of the cooking pot before the water and other ingredients are added. Eventually the boil over preventer will absorb any excess heat build-up created by an unchecked layer of foam. As long as the ceramic or glass boil over preventer is in the water, it should not be able to reach superheated temperatures and boil over the side."