Angie Taylor posted a quick tip on Wrapping text around a circle in After Effects.
There's another intro tutorial on the "newer" way to do path text here at AEtuts, Animating Text Along A Path In After Effects – AE Basix, and some addition details at the AE Help page Creating and animating text on a path. And Eran Stern posted a more complicated video tutorial, Tornado Text, that shows how to use per-character 3D text animation to animate text along a path in the shape of a 3D tornado.
The "old" way to do text-on-a-path was with the now defunct Path Text plug-in. It was very easy to use, and if you are working on a project that was created in an older version of After Effects and the Path Text effect was applied, CS5 lets you continue to use the Path Text effect. By the way, the first vertex of a closed mask affects control of mask paths.