"I read your tutorial on how to sync Outlook Calendar and Contacts with Gmail and the T-Mobile G1, but you didn't cover syncing your Outlook mail with Gmail. I've got about 50 folders and 5 years of mail in Outlook. How do I get all that old mail into Gmail and keep both in Outlook and Gmail in sync?"
I didn't originally cover how to keep Gmail and my G1 in sync because at first I was attempting to continue using Outlook as a POP3 email client, while also accessing my mail from the Gmail Web interface and from my G1. This was a horrible idea from a time management perspective. Outlook treated all messages downloaded from Gmail as new mail, even if I'd already read it online or on my phone. The solution takes a couple of steps that require an initial upfront investment of time, that pays huge dividends later.
The most important part of the process is to stop using Outlook to access your email via POP3 and start accessing your email via IMAP instead. To do this, sign into your Gmail account and click on the Settings link up in the top right corner of the screen. On the Forwarding and POP/IMAP tab, enable IMAP and be sure to disable POP3.
In Outlook, you will want to go to Tools > Account Settings and eliminate any existing POP3 reference to Gmail. You will then create a new IMAP account. Once this is done, you want to copy all the old folders in Outlook to your IMAP account, which transfers them to your Gmail account. If you have a bunch of mail, this takes an extremely long time. I detail this process in another post on moving Outlook email to Gmail.
Using IMAP gets you the automatic sync you need. So if you read a message on your G1, it will already be marked as read in Outlook. The folder copy process takes all those old messages and makes them accessible via Gmail.