I had one other thing like this happen before. When I blew out my Catalytic Converter it came out in the form of a fine white powder. I would stop at a light and wait for a minute, then hit the gas. Fist thing that happened is a white puff out the back.
Check to see if the white smoke is either:
a) water vapor such as coolant, oil, etc... It will most likely leave the inside of the tail pipe damp and will probably condence on something held right behind the exhaust.
b) powder, I actually held a piece of cardboard behind one of my exhaust tips while my friend reved the hell out of the engine. This created enough flow in park to kick some more "white stuff" out the back. A few pieces came out just large enough to recognize the honeycomb shape of the insides of the cat.
Maybe one of those ideas will point you in the right direction.