My only issue with whiteboards is that many people don't know how to use whiteboard pens; I've seen many professional mathematicians take off the lid, write a sentence, then talk with the lid still off. A whiteboard marker should last a very long time; certainly throughout a full talk with only the one marker.
On those grounds, I prefer blackboards. But if everyone recapped the lid automatically the moment they were done writing, even if just for a few moments... then I wouldn't really care that much.
