Assuming this post makes it and you can read it, I think I fixed the problem.
It looks like the messages table somehow got corrupted.
Initially I thought it was related to the SMF upgrade, but it looks like that had nothing to do with it. The upgrade doesn't even touch the db.
Anyway, I restored the latest backup (from 4am or 5am this morning) and "repair"ed the table and it seems to be working so far. Let me know if you have any more issues. Unfortunately any posts made after the backup script ran are lost for good.
My email is psidefect AT gmail DOT com.
Save it. It's almost always the best way to get in touch with me. I don't want to post my cell number here, but if you PM me, I'll gladly give you that info too.