Google have just made Gmail available offline as a lab feature. It relies on another Google technology called Gears that theoretically enables any web application to also work locally without an internet connection. This can be very useful when using a dodgy connection or not having one at all. Gmail will then synchronise automatically when it can reach the servers again. I just enabled the feature and downloaded Gears on my Linux laptop to give it a whirl. I will install it on my Mac OS-X desktop later to see how the two platforms compare.
As it is still experimental, it is currently only available for some English US and English UK users. So if you don't have it available yet, be patient, it will come your way soon.