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I'm sure everyone reading this, especially those of you that use Gmail, is aware that IMAP is now available through both Gmail and most Google Apps accounts*.
My Google Apps account became IMAP-enabled on Friday, November 2nd, 2007. Once I saw this I quickly moved into action to setup IMAP on my iPhone and in Mail.app (Apple's Mail client on the Mac OS). I followed the instructions in Gmail's help section to the letter. First I setup my iPhone and everything went swimmingly. Then, I moved all of my email from my Inbox into an archive folder in Mail.app (in case something went awry) and setup IMAP in Mail.app. Once I hit save, Mail.app went out and grabbed all 4,000 messages from the Gmail server.
The email address that I'm setting up is my personal email account that I've only had for a few months, which is why there was only 4,000 messages to sync. To date, I'd been using POP with Gmail and so all of the messages came in marked as 'unread'. I thought nothing of selecting all of the messages and clicking "Mark all as read". This led to four days of Mail.app syncing with Gmail's servers before I was able to receive a single new message. Very frustrating.
I'm not sure what I could have done differently to save myself from this four day hassle. Perhaps logging into my account through my browser and marking all messages as read there first? Either way I am definitely happy it is over and if you find yourself in a similar situation to mine - know that there is light at the end of the tunnel.
Derek Punsalan has some really great tips for setting up Gmail's IMAP on both the iPhone and Mail.app that go beyond what is provided in Google's help area. I recommend that you make these small changes to your setup, which seem to make things much much nicer when dealing with multiple devices.
* I say "most" because it seems to be that gmail.com users got IMAP quite a bit sooner than Google Apps accounts and that some still haven't had it enabled for them.