My wife's 2003 Grand Am has two levels - light on solid means you can keep driving until you get it looked at, flashing means pull over immediately. I've only seen it flashing once - when the engine stripped a rocker arm and started flailing bits around in the head. This is an OBD II standard across all cars with the system.
Incidentally, they sell a cheap ($20, and often $15) bluetooth- or wifi-enabled plug that links up to a cell phone app to read codes and other OBD II info. It's been a godsend when I was trying to diagnose a couple issues with that car.