Airturn is a great solution for musicians who use sheetmusic on their iPad.
It gives you the possibillity to use your feet instead of your hands that you're using to play your instrument.
There are alternatives, but they are not so popular.

Airturn has a sample application for developers, but it did not work as wished.
So, I made one myself and made it very easy to implement. See the wiki for instructions.

So, how does it work?
Airturn functions as a bluetooth keyboard, not more than that.
Apple does not make it easy. Since we cannot read a keyboard directly we have to deal with a UITextField. 
But you don't want a virtual keyboard appear when you disconnect the airturn, and the api can't tell you if you're using the virtual, or the external keyboard.
So we have to make our "own" keyboard, by assigning the inputview to the UITextField.
Since we don't want a keyboard, we make an UIView of 0x0 pixels, and assign it as our keyboard.

If your happy, and want to give me a coffee:
Donations are welcome on my paypal: [email protected]
Thank you