Well this should bring some smiles to the community if we can make this work. I've been trying to figure out how to get push email working on my iPhone with my own domain. Very long story short, Yahoo stinks because its all POP based, GMail doesn't support push (aside from imap idle which iPhone ignores) and MobileMe doesn't let you change your from address and forwarding is ugly anyways.
After some searching on Google I came across a little gem called Z-Push. It is a PHP app that appears to integrate with a maildir structured server to provide push email using ActiveSync. This would essentially offer push email to every account hosted in Virtualmin and would support iPhone 2.0, Windows Mobile Devices, and Series 60s. I'm not sure if this would best be implemented using server scripts and then enabled for each domain or if it would be better to setup one instance in apache and setup aliases from each domain. Eventually maybe some sort of shared address book / calendar could be integrated as well but I think push email would be a great start.
Project Homepage: http://z-push.sourceforge.net
Latest Code: https://developer.berlios.de/project/showfiles.php?group_id=8963