This is interesting, though it's obviously geared only towards enterprises at the moment:
Not long ago, we launched an email migration tool that lets administrators move email from their existing IMAP mail systems to Google Apps, so users didn't have to leave their old mail behind. Since then, more than a hundred million messages have been migrated. But this solution didn't solve the whole problem. If your existing solution didn't support IMAP, you were out of luck.
Today, we're excited to announce the new Google Apps Email Migration API, which can move email from anywhere – not just IMAP systems – to the Premier, Education or Partner Editions of Google Apps. If you need to migrate email from your users' desktops, migrate email from a server not supported by the IMAP migration tool, or do a push migration because of your security policies, a tool built on this API could be the perfect answer.This API is just the latest in a stream (read, torrent) of innovations we've rolled out this year for Google Apps email.
Related: Google Apps Email Migration API Developer's Guide