My initial reaction to this was less than enthusiastic--I mean, the very notion of uploading several years worth of email to Gmail, it just curdles the blood. But the more I think about it ... Yeah. I have to do this. And if you've made the transition to Gmail, as I have, you may want to think about this as well *.
Google unveils the Google Email Uploader:
The Google Email Uploader is a open source desktop utility for Microsoft Windows. It uploads email and contacts from desktop email programs (like Microsoft Outlook) into your Google Apps mailbox. It preserves information such as sent dates and sender/recipient data, as well as the folder structure used by email programs.
* Note that it only works with the Premier, Education, and Partner versions of Google Apps right now, not the standard Gmail service. Hopefully, that will change at some point. On the client side, it appears to support Microsoft Outlook 2003 or 2007, Outlook Express 6 (but not Windows Mail), and Thunderbird 1.5+, and Windows XP and Vista.
Related: Gmail Mail Fetcher (download messages from up to five other email accounts, centralizing all your email in Gmail)