A while ago, I received a letter from PG&E mentioning their Smart AC program. This program lets customers voluntarily let their air conditioning get throttled when there is a significant energy demand.
The box that is installed on the compressor, has a pager network radio receiver, and when it receives a signal from PG&E, it will throttle the AC back. It looks like it does it's throttling by cycling the compressor on in 15 minute intervals, when the thermostat tries to turn the AC on. During this time, since the thermostat thinks that the compressor is on, the fan will still be running to help with the cooling of the house. Users also have the ability to opt out of the throttling in 24 hour periods.
In our case, I don't think that we will be affected, as we hardly have the AC on. But, I figure that we can do our part, and if turning off our AC in 15 minute intervals, can prevent a black out, we can join the program.