Hold To Toggle

Hold on any conversation in Messages to mark it as unread (or read, if it's already marked). This should be the release candidate for BigBoss.

Unread: Hold To Mark

Hold on any conversation in Messages to mark it as unread. Every hold adds another "unread" count, so, if you want, you can mark every conversation as having hundreds of unreads. Opening the thread marks it as read, just as usual. Will most likely become a commercial tweak ($0.99?) in the near future.