On the EchoDitto Labs blog there's a new post about getting Eclipse PDT with Zend Server Debugger on MAMP for Drupal (by Jeremy John).

Anything involving Eclipse is always epic. First, you have to get your head around what distribution of it to use, as confusing as one's first introduction to Linux distributions (there are different kinds of Linux?). Next, you have to grok the fact that the Zend Debugger must be installed on your server. In this case, MAMP. Then, you have to make Eclipse listen to the debugging information being outputted, on the debug port. Finally, you have to figure out how to use a debugger effectively (not in scope, but careful, debugging will blow your mind).

After following along with this screencast he was still getting errors when trying to debug (connection failed message). Eventually, though, he finally figured out the problem - an extra zend_extension call to load the XDebug module.