There are still a few limitations. This quickcast contains an updated overview of the current (3/18/11) limitations and the advantages of using your iPad as an interactive whiteboard solution. The limitations are shrinking as the advantages grow! The iPad as an IWB is really an awesome, useful solution! I know of one school district that is already discussing implementation.
Having trouble viewing this quickcast? It is also cross-posted at my YouTube channel: iUpgradeTV. The URL is: http://www.youtube.com/user/iUpgradeTV