A buddy of mine works at one of the Apple stores here in the OC. They actually use standard Expo dry erase board cleaner. They said it is actually the best solution out there they have found thus far.
sometimes they use iKlear (which in my opinion is the best to use), havent tried Expo cleaner...dont see why it'd hurt.
BUt the apple stores in my area stated they use Monster Screen Cleaner as they have a deal with them or something of that nature.