Besides fuses, modern circuits are generally designed with a failsafe to detect reverse current (read: diodes, reverse current detectors, etc). You flipping the polarity generally won't do damage to a modern circuit (read: car). However I sure as hell wouldn't recommend it. I've fried more than a few chips reversing my currents on homemade circuits. Though even with basic components (read: integrated circuits), there are failsafes in place.
--Diamond