Official Gmail Blog: New in Labs: Offline Gmail
Here's the posting on the Google Blog. I still have not seen it on my account.
Wonder how many days it will take to show up.
We're making offline Gmail available to everyone who uses Gmail in US or UK English over the next couple of days, so if you don't see it under the Labs tab yet, it should be there soon. Once you see it, just follow these steps to get started:
1. Click Settings and click the Labs tab.
2. Select Enable next to Offline Gmail.
3. Click Save Changes.
4. After your browser reloads, you'll see a new "Offline0.1" link in the upper righthand corner of your account, next to your username. Click this link to start the offline set up process and download Gears if you don't already have it.