Just installed Ajax Comment Preview. Works like a charm, not a problem of any kind. The format options take a minute to figure out, but easy to set up. Many thanks to the author for doing such a nice job. The preview opens up properly, then closes with a screen refresh when you click Submit. Puts you right back where you were before you used it, which is how all tools like this should work. High marks.
One suggestion: Please offer a format that puts the previewed text in a box to set it apart more clearly from the rest of the comments section around it. What would be really cool is a Flash implentation that creates the "floating Flash window" we've all come to know and love.
But beggars can't be spec writers. ;-)