I've noticed this a couple different times, both during and immediately after driving the car fairly hard, I get this smell like plastic burning. It's not burned rubber from the tires because I rarely light them up, this smells like melted or burned plastic. I noticed it yesterday when I stretched it out on the interstate, I could smell it inside the car, and when I pulled in the garage it was really strong around the rear of the car. Could it be the catalytic converters sort of burning the "new" out? It's a 2006 M6 with about 2100 miles on it.