Ive done it... but usually it is an exotic car. I think if its obviously a "car person's" car... then they will understand. But if your taking pictures of peoples toyota prius... they might call the cops
just be respectful and don't try to make a photoshoot out of it. give the owner and car the respect it deserves as a personal piece of property, not some thing to be ogled at (though maybe that's why the owner has that car in the first place).
case in point - i saw a 1972 Lotus Europa in a work parking lot during a lunch break, so i went over and snapped a couple quick shots on the cameraphone. owner was walking towards the car anyway, so i asked about it, and he offered to take a pic of me sitting in the driver's seat.