No.<p>Facebook messaging is like email... and if you get a lot of messages, subjects can be helpful.<p>The comparison to Twitter doesn't really make sense. It is Facebook status that is like Twitter (and has no subject requirement). Facebook's inbox is not like Twitter.
So whip together a Greasemonkey script and put the choice in the hands of the savvy user. Not the most elegant solution, I know, but it's clear there's a difference of opinion so taking either road would leave someone out in the cold.