http://userscripts.org/scripts/show/137452 may help as well.
 
I am not making a meta about it, yet, however, until I get around to adding a preview and something for options.

Textile tutorial post would be fantastic. I’ve lost all the tricks I had with BBCode, and the Wikipedia page for textile isn’t terribly informative.

Yes please!! I need to know all the textile tricks.

I could make one up real fast. I have to admit, the Textile help presented below the comments is a bit bare. I’ve been experimenting myself and there’s more you can do.