I have an app that was started in Mx4 and has since been upgraded to Mx5. In looking at the app platform, I see that my acceptance and production databases are both Postgres 8.4, while other apps that I have which were created in Mx5 have Postgres 9.2 databases. I am wondering if there is something I need to do to upgrade the database in my app to Postgres 9.2, or is this something the app platform will do without my intervention.
To be honest there's no hard link between mx4/5 and postgres versions. The reason you're seeing different versions is purely coincidental: new nodes get created with newer versions of postgresql. We mention the versions to help you in case you run into any weird issues and so that you know what version of postgres your backups are.
We'll be migrating 8.4 postgresql nodes in the future to 9.x, if you have nodes running 8.4 you'll be notified. If you already want to migrate now you can file a ticket for that.