The latest version of OnSong has just been approved by Apple so you should be getting an update soon.  This one is very stable and usable from what I have found so far. I was also able to send it out to some early adopters and have had great feedback.  Now on to the next version.

http://itunes.apple.com/app/onsong-for-ipad/id364493059?mt=8

Note: it sometimes times up to a day for the version to become available so be patient!