My first Cake Central Post! :)
I make a lot of cakes covered in fondant and I have yet to figure out how to get rid of powdered sugar marks (or if a drop of water gets on the cake and I use powdered sugar to dry it)--sometimes I use vodka or water to "paint" the entire cake because of the dull look or powdered sugar areas, but that typically results in paint brush marks when it dries.  Will try steaming, but not sure if that will do the trick?  Any advice would be much appreciated!

I avoid the issue altogether by rolling on shortening--no mess at the start and no fixes necessary at the end..

You can steam the whole cake or mist it with Everclear or vodka.  It helps, but if it's really covered, it won't do a lot of good. 

Water melts the sugar and makes shiny spots.  If I get a dot of water, I hit it very quickly with some cornstarch and then use Everclear to get rid of the white spot.

I have a high quality soft paint brush that I use to brush 'dust' away my powdered sugar or cornstarch that gets on the fondant. I use it dry. I live in a very humid south Louisiana, so I most often roll out on cornstarch to help combat the humidity.