What does this mean?
It means you can read your email from your Gmail account directly in most email clients (Apple’s Mail.app at home and Mozilla Thunderbird at work, in my case) as well as from iPhone without the need for any plugins or third party software – and your mailbox stays synchronized all the time no matter where you access it from. This is something I’ve wanted to do for years.
This way, if you’re in front of your new and shiny MacBook you can use Mail, but if you are traveling or don’t have your computer with you, you can keep using the web interface. Not bad
Google answers some questions you may have here.
[Update 10/29/2007] Today I was finally able to enable IMAP in my Gmail account, but neither Mail.app nor Thunderbird work with it. I’ll let it rest for a couple of days to see if it’s something Google needs to sort out on their end or if I screwed up on my side.