Added a preview button. (Just on the text boards for now.) The javascript to shove the button and form onto the post is a bit unwieldy for what should be such a simple task, but hey, that's javascript for ya. The server handler, on the other hand, was stupidly easy:

# -*- python -*-
elif 'preview' in form:
    message = handle_message(board, form.int('preview'), 0, form.field4, form.markup)
    io.write_http_header(length=len(message))
    sys.stdout.write(message)

_{No, that form.markup field isn't meaningful... yet.}