One of the best features of iOS 5 on the iPad is the ability to project your iPad 2 screen wirelessly to a display via AppleTV. This is accomplished via Apple's Airplay technology. The AppleTV has an HDMI connector so, basically you just plug your AppleTV into an HDTV and your can wirelessly project your iPad 2 screen on the TV. 
This is great if you have an HDTV, but most classrooms do not. This is where the HP HDMI to VGA converter comes in. It is a small attachment that has an HDMI connector on one side, and a VGA adapter on the other. This allows you to plug an AppleTV into a data projector that does not have an HDMI connection. The connector will pass the video, but audio will not pass through. We only have one projector in our building with HDMI and ideally this is the best way to do this, since you get video and audio, but for most of what teachers want to share, being able to pass the video image will be very helpful. The HP HDMI to VGA converter is about $40 and available from HP. An AppleTV will set you back bout $100.