Rick Gerard at AE Tips and Tricks has a video tutorial on using the built-in After Effects filter Shatter to extrude text. For now the link has a typo, so in the link replace the "c" with a "d."
It's not a complicated procedure but I don't remember seeing a tutorial on this since Brian Maffitt originally released the filter. There are several as it turns out though, like Roland's Adding Z-Depth to your Logo at Creative Cow, at Adobe Exchange by G. Chen, at Video Copilot, and Maltaannon.
Update: on the AE-List Roland Kahlenberg added, "If you're using Shatter you'll want to apply the effect on a Solid that is twice as large as your final output. Then scale it down accordingly. The scaling down reduces the aliasing issues which may become obvious on certain types of geometry."
Update 2: There's detail and other more references on the Shatter filter in AE Help.