Whether this is new or not I'm not sure, but it was news to me.
I spent quite some time with six stars driving in circles dropping grenades on the cops. Pretty awsome.
Here's the vids (not mine)
All you have to do is take a girlfriend (they all work except for Michelle) out on a date. When they get in your car, start shotting/dropping grenades etc and the date will end. After that the car you are in is indesructable, and you can even save it at your safehouse.
You can only do this to 2 of your saved cars, if you try to make a 3rd the other 2 lose their EP properties.
It works for almost all vehicles, but not all of them. It doesn't seem to work on mission-related vehicles or some cop vehicles.
Cars become indestructable, even the glass and tires become bullet proof. Tires can however be burnt off by fire.
Veiwing a cut-scene while in the vehicle will remove the EP properties.
The cars wont be damaged by collisions, although collision damage will show. But you'll never start smoking, and it won't blow up, and no matter how screwed up the car gets it'll drive fine.
You can still get launched out the window so play safe.