I have been using Coda and regular version of Mamp for local development for longtime without getting into this permission mess. I recently, upgraded to Mamp Pro and setup it with VHosts. I have a site example.com with it’s root path set to /Users/john/Sites/example. I have set the owner and group to www in Mamp Pro.
The moment I got all this configured I started having problems with Coda. It keeps asking me the username and password to edit the local files at /Users/john/Sites/example. I guess I have to enable FTP on my Mac and then add a site in Coda to stop it asking me to enter username password for every single file. However, I have no idea on how to get this working. I am using Lion 10.7.2
Additionally, I have setup etc/hosts file for pointing example.com to 127.0.0.1
UPDATE: Though the accepted answer by @mini does not directly answer this question. It is still an elegant solution with seamless integration with Coda.
Consider using DesktopServer instead (along with Coda 2). Unlike MAMP, you can work on template theme files directly with WYSIWYG preview, LAN share for mobile device testing (with WordPress, -not just HTML sites), enables AirPreview to work with WordPress, copy, import/export to live sites, etc. Setup is easy as it manages your vhosts, database, and project files in about three mouse clicks:
http://www.youtube.com/watch?v=Pw9-F8etBPY