By setting the address book preferences to synchronize PowerMail's address book with Apple system-level address book, you ensure that MobileMe inherits all changes made to your PowerMail address book either manually of via filters.
Additionally, PowerMail has been found to coexist well with iPhone mail; a simple strategy for this coexistence is to use PowerMail's strenghts in managing and searching mail received using the POP3 protocol, while using the iPhone with IMAP to peek into the the same e-mail account's Inbox. This way, all e-mail already fetched by PowerMail will not need to be downloaded on the iPhone, while iPhone-read mail will indeed get fetched, stored and indexed in parallel by PowerMail.
An extra benefit of this technique comes in using the iPhone's excellent interface for quickly deleting several messages over IMAP without having downloaded them initally; this way, any spam spotted and deleted on the iPhone never gets fetched by PowerMail.
Finally, PowerMail's lightweight-IMAP implementation allows occasional Mac-based management of the MobileMe or other IMAP-server based mail queue.