Is there an iPad app for taking screencasts?

Yes, there's a third party app called DisplayOut, but sadly, it requires jailbreaking your iPad, which I do NOT advise doing. Jailbreaking your iPad will make it unstable and crash-prone (a.k.a unusable), so I wouldn't consider that as an option unless you dedicate it solely for making screencasts and use a second iPad for your primary device, which I doubt you want to do.

Alternatively, if you have a decent HD video camera and a tripod, you can use it to record your iPad screen. This method works best if you record inside a dark room. Lower quality camcorders will produce less-than-ideal results.

Lastly, if you're a developer and you happen to own or have access to the specific iPad app build, you can use the iPad simulator on your Mac and use screen capture software like Camtasia to record your screencast.