I've seen farm tractors do this on ocassion, usually with a straight pipe, but never a vehicle.
It was 27* this morning and I could see a few in the condensation as it was coming out of the tailpipe. The truck was idiling as I was letting it warm up a bit before hitting the street. It's a 79 F100 302, Ford Racing headers, Flowmaster 3" muffler and single 3" full length tailpipe, exiting behind the rear wheel.
I've been driving this truck since 1984 and this is the first time I ever seen it do it. It just turned 300,000 miles last week. The engine doesn't smoke, and I only noticed it in the condensation that cars usually put out in cold weather. Does anyone know what causes this, or do I just have a talented truck?

 

I think it has something to do with the timing of the exhaust pulses... I've heard it's common with vehicles with dual exhaust...