@MarkM: The motor that runs the compressor will wear at the same rate whether the gas engine is on or not. I can’t imagine that this is any less reliable than a belt-driven compressor such that non-hybrid cars use.
You will use the same amount of energy either way. As others have mentioned, when the battery gets low enough the engine will run to make up the difference. The energy has to come from somewhere. If you are using the A/C, it is using the same amount of power whether you are idling or driving. If you have a full battery when stopping with the A/C on, it will take a little longer for the engine to start, but when you begin driving again, you will have used the same amount of power and the engine will come on sooner. If the battery is nearly depleted when you stop with the A/C on, the gas engine will run, but again, the A/C will have used the same overall amount of energy.
Whether “idling” a Prius or just another compact car with a similar-sized engine, you will still end up using the same amount of energy to run the A/C, assuming the A/C on both vehicles is equivalent in efficiency. The only difference I can see is that you might also be charging the battery and can use that energy when driving.