@glasshouse: There are a number of different causes of meltdowns. What you described is a loss of coolant accident. This is the type of accident was part of the cause of the Three Mile Island partial meltdown. From what the news is reporting, this is also the cause of the Fukushimi reactor meltdowns in Japan.
I am not an expert in nuclear chemistry, so I am not sure exactly how it occurs, but power spikes in a nuclear reactor can also cause meltdowns. The Chernobyl meltdown was caused by human error that led to a power spike that damaged the reactor core.
Problems with pressure can also cause meltdowns of nuclear reactors. In these cases, coolant pressure drops thus reducing the heat transfer efficiency of the coolant. The result is the overheating of the fuel rods and a subsequent meltdown.