Dustbury has a post up about a study of the phenomenon of time stretching during crisis situations, and relates some of his own memories involving the kind of crises that happen at the helms of motor vehicles.
From my experience, I have to say that I agree the phenomenon is almost definitely one that is generated in memory, rather than something that actually occurs during the incident. I've never been in an accident in a four-wheeled vehicle, but I have had two depart from controlled flight underneath me (an old Dodge Dart that swapped ends on a busy five-lane thoroughfare when the secondaries cut in in a tight bend and a Fiero that ground-looped several times through a crowded, rain-slick parking lot) and one near-miss (an oil slick that had the Zed Three fishtailing half the length of a freeway on-ramp while I sawed wildly at the wheel) and all three of those incidents are still available in excruciating technicolor in my mind's eye, despite the earliest one being nearly two decades old.
What makes me convinced that this is a function of memory rather than something that happens in real time during the actual event are my experiences on motorcycles. All the countless near-misses are still there in memory, suitably dilated in time, but the ones without happy endings are shockingly brief; one minute you're riding along and notice something about to be wrong and then *BAM!*, the rough hand of physics smacks you to the asphalt like a broken doll and there's almost no how-I-got-from-there-to-here involved. Like the sticker on my old helmet says: "It went earth, sky, earth, sky, earth, sky, earth, ambulance."
A guy I once knew who played football through his scholastic career described hard tackles the same way. He said that the ones he could see coming seemed to last forever, but when it was a blindside hit he went from standing up to wondering why his helmet was full of grass in the blink of an eye.