Not for iPad, but for desktop/laptop Mac or PC...
I use the combination of Rhyme Genie ($25) and its companion Tune Smith (free). I don't know if they'll ever make a mobile version. That'd be almost too good to be true.
http://www.rhymegenie.com/index.htmlhttp://www.rhymegenie.com/tunesmith.html I've gone through a lot of different writing tools, and really like this combination. It's more tightly integrated on the Mac, but the PC version works great. You can manage your creative talent, song catalog and pitch journal right from Tune Smith. It's amazing for a free add-on.