I recently attended a great IDOE regional conference at Center Grove, and saw many new Apple TVs in use. The Apple TV is a great device with a multitude of uses in the classroom. However, if your main goal is to find a way to wirelessly show your iPad 2/3 or iPhone 4s content on your projector, then there is a much simpler and cheaper solution. The Reflection App ($14.99) can be downloaded and installed on your Mac or PC, and it does exactly as its name indicates - it reflects your mobile device's screen onto your computer.
I stumbled across this app a couple months ago, and I was sure that it was too good to be true. Luckily, the company provides a free trial period, and this was enough to convince me it was the real deal. With your computer and your device on the same wifi channel, you can launch a video on your device and the video AND sound will play on your computer. I have really enjoyed using this for my Keynote presentations, and also for workshops where I am training on how to use specific apps or a workflow on the iPad. The Apple TV ($99) has uses far beyond mere reflection of mobile devices, but if this is all you need in your classroom, then the Reflection App could be your best choice.