It starts to be a reality using the Gmail in off-line mode. Gmail uses Google Gears to store the relevant information synchronizing the information whenever we have an Internet connection. This allows us to do simple things such as reading the mail and sending emails. Of course, the effective sending of the mail will only be made from the moment we have a valid Internet connection. I can only say that this new feature will be very useful, the only penalty for now, is the fact that it is not available in Portugal. It's only available in the U.S. and UK.
See more at Official Gmail Blog: New in Labs: Offline Gmail.