Not that this is a very timely reply to your post but I have had numerous diesels and now a S420 and the only thing I can think of is that there is a vacuum leak somewhere in your system. In the diesels the default for no power and no vacuum is full defrost with full heat.
I have noticed that in mommies 99 S420 that occasionally in the morning when pulling away from a stop it seems to shift over tot he defrost and then back to the footwell vents. I have not dug that deeply but my assumption is that the vacuum in these is supplied by the manifold and not a vacuum pump as in the diesels therefore it could be due to a temporary loss of vacuum on acceleration that makes this happen.
I would think though that MB would have used a vacuum resevoir on these cars to eliminate the temporary loss of vacuum incurred during full throttle acceleration.
Sorry if this doesn't help much but I am just starting to explore this S class gas thing...
