I’m happy to report a PHP counterpart to imap2soap is coming in CiviCRM 2.2, so the handling of CiviMail’s return channel should be much easier in the future (ideally as simple as setting up a cronjob).
There are also other CiviMail improvements and changes in 2.2 (kudos to U.S. PIRG for their sponsorship!) which I plan to unveil in upcoming post(s), but let’s concentrate on the return channel for now.