With the aid of Google Gears, Gmail users can now work offline with their email account.
Gmail is being downloaded locally and continuously synced with the Gmail servers while there is an online connection.
Then if you go offline, you can still view your email messages, write new messages, label, star etc. And the next time you get online, everything will be synced again.
The offline option is currently being rolled out, and you will eventually find it in Settings -> Labs.
Here is a small presentation video from the Official Gmail Blog: