I got vi.js from: http://gpl.internetconnection.net/vi/, modified it for my needs, and voila! A bookmarklet! Add it to your bookmarks, go to a page with a large unwieldy textarea, pull down the bookmarklet, and away you go! I'll add a selector for pages w/ multiple textareas one of these days.

Launch FormVi    <-- bookmark that link

or, just try it on this form: