Message Boxes in OpenERP

Edit: This technique does not appear to work in Odoo/OpenERP 8

Being able to show a user a message is a pretty basic, important piece of functionality. It took me a while to figure out how to trigger a user-visible message from the server in OpenERP, but I eventually managed it. Given the answers on Stack Overflow, I figured I should share this valuable finding with the rest of the world.