Oh, well.
I just spent an half hour researching and writing an educated reply.
OS/2 World/Firefox ate it, so that's the end of this thread for me.
Sorry, but that's way it sometimes goes.
Martin, I have "stayed logged in" checked, but that doesn't solve the problem.
That it did give the poster some more time on the old forum, but something has changed in this incarnation; you might want to check the "cookie settings" in the SMF forum server settings and/or in Joomla/Mambo settings. I have cleaned out and reset the cookies several times in Firefox; it changes nothing.
Other users here have complained of the same issue. Paul Smedley is aware of it too and related it to a known problem with PHP. It might not be fixable.
Usually I can pull my lost message out of the cache when this happens, but not this time and I cannot afford the time to rewrite it.
This post is just FYI.