A good guide is http://www.squishychicken.com/index.php?option=com_content&task=view&id=13&Itemid=2. It's for AMP and Asterisk 1.2, but can used to install freePBX instead.
Updated for FreePBX and other software version changes. I also felt it needed to be made more idiot proof for dummies like me who spent many frustrating hours trying to get it to work.
http://powerontech.com/freepbx-on-debian.htm
There is a good Ubuntu guide here that can be of assistance, until somebody writes an updated guide for freePBX 2.3 and Debian Etch.