If you've kept your old emails around (woohoo gmail!), one thing you can do is go back and look for the notification email you got for a particular post that you suspect was a message. There's a date search widget that makes this easier. I suspect you'll find it was a wall post.
Another thing to note (for those that follow our technology), is that the backends for these two systems are entirely different. The messages backend is hbase-backed, designed for real-time communication more than history. The timeline backend is MySQL / C++ backed. Migrating data from hbase to MySQL would have to take months of effort.
I know that even though there was no bug here, this can be an alarming experience to see old posts surfaced like this. We're working on ways to make this interaction clearer so people aren't so surprised in the future. That said, please rest assured that your inbox is not on your timeline.
You ought to be doing more than that, actually. Having been an early user of Facebook, I can assure you that people viewed wall-to-wall and other types of posts differently from how they view wall posts now. No one expected their data to be exposed this way, so for practical purposes, this is tantamount to a bug or data breach from the user perspective.
Disabling this immediately for old posts or allowing users to opt out would be the right thing to do.
Personally, I'm sad that all of my private messages have been so mundane that if they got out, it would be no big deal. I just looked through and I didn't find any meaningful dirt at all. What a shame.