Symphony Pro. A very capable app for the iPad allowing me to write music anywhere, anytime. The developers have cleverly implemented the touch screen interface in a quite intuitive way to get the dots on to the stave with a finger. Alternatively, you can use the internal keyboard or an external keyboard, which I have done using my Yamaha S90XS with the Apple camera connection kit and a USB cable from the To Host USB port.
Incredibly, you can import pdf, jpg, gif, png and bmp files and the experimental sheet music scanner will have a crack at converting it into an editable MusicXML file. At this point in time it is not always successful but does put the file in to a format that allows you to then send it to Sibelius or Finale for further work. As I use Sibelius the Dolet plug-in needs to be installed to read the MusicXML file.
As well as MusicXML, you can export in PDF (B&W and colour), MP3, AAC, MIDI and as a Symphony file to share with other Symphony users.
This app is new so it has frozen on me a couple of times but the developers are resolving these issues very quickly. The app costs $15.99 here in Australia and is worth every penny. When I first got an iPad 2 I had no idea how much I would be using it in my musical life but it has already become an essential tool as I keep many of my scores on it using OnSong and now it has become even more useful!