Going by the wife's Honda Insight, there can be many operating modes. Depending on the ambient temp, if the driver has "econ" or "normal mode" selected for the HVAC control, the state of charge of the batteries, if the computer detects you are in "stop-and-go" traffic or not, and probably a few other parameters I have left out. Probably a different list for the Saturn, but still many different modes it can run in.
We just brought home a Mariner Hybrid for the wife today. On it the engine does stop at strange times like when you are driving around a parking lot or sitting at a stop light. The book says that it might stop anytime the car is below 40 mph.
You really can if you keep you foot off the pedal keep the engine off and it sounds just like an electric golf cart. And the computer already says that we are getting 29 mpg after about 50 mi.
Takes a little getting used to to drive but I think that we are going to like it. The 3K back from the gov this year will be nice to and moving from a V8 Mountanieer will help monthly with the gas I hope 15 - 34 stated in the city 20 - 30 highway. My wife wanted another SUV and this is better than most cars.
Also the engine is rated for 10K between oil changes with full synthetic so that might be a wash.