I too have been searching for a FoldingText app for iPhone. I much prefer the markdown syntax of FoldingText to Taskpaper. The Editorial is the closest that I have found so far. I rename my files to have the .taskpaper extension.
I then take all of the Markdown style # headings and add a colon : to the end.
## My Heading
## My Heading :
This goes a long way to making the TaskPaper mode handle Markdown files. I hope that Editorial will eventually support FoldingText as a mode within markdown documents.