actionreaction

I was having the same issue - white smoke when under hard acceleration or downshifting and today on the freeway things went bad. Turns out there was a transmission leak that would increase under significant acceleration. Fluid would hit the cat converter and pipes and evaporate quickly resulting in the smoke. It looked like it was coming from the tailpipes but was actually coming from the undercarriage.

A seal or something finally blew today and the car is in the shop. Tranny may need to be pulled.

If you're seeing the white smoke under the same conditions, have your shop remove the skid plate over the transmission and exhaust to see if there is staining on the pipes or cat converter from the leaked fluid burning off.

DESAL

Read these two links. They gives all the possibilities. White is not the best thing to have.

http://www.dieselsmoke.com.au/

or this one - more technical

http://www.eng.wayne.edu/page.php?id=754

actionreaction

I don't see how these are relevant. The 550 does not use a diesel engine.