Why not just use a normal textarea, with a BBC toolbar like Minorin, but have a live preview via jQuery of what your typing? I see it being done with the topic subject line, why can't you do that with a whole post?
This would also eliminate the preview button, as preview would be automatic and live. The only exception would be for quick reply, but a live preview wouldn't be needed there. This would also help user's understand how to properly use BBC.
Just look at this: http://pushingkarma.com/projects/jquery-wysiwym/
It's even compatible with iPad...