Jeff, thanks for your reply. I figured it was something like that but was never quite sure.
Now that I think of it, the smoke/steam only appeared when the car had been hot. On those occasions when I washed it just after starting it for the first time in the day, or after it had been sitting for a few hours, there was no smoke, so your condensation explanation makes a lot of sense.
The only problem is that it is embarrasing to be spewing smoke like that -- even for a short while. Other drivers seem to get some sort of perverse thrill from seeing a disabled or malfunctioning Benz.