Gmail Labs has released a new feature called “Offline Gmail” as announced on official gmail blog. Offline gmail uses Google Gears to download all your emails to system cache when you are connected to network. Offline Gmail keeps syncing the local cache with server when connected. When you are offline, turn browser into “Offline Mode” by going to “File–>Work Offline” & visit gmail.com as usual. All the emails will be accessible as usual.
To enable this feature goto “Settings–>Labs” & enable “Offline Gmail”. You might have also required to install Google Gears to get this functioning. This feature is not yet released to all accounts yet, but will be releasing soon. Keep looking at your Setings page.