An experimental feature has been added to Gmail - offline access. Using Gears, you can now download a cache of your mail, for offline viewing. You can then also compose email and it is stored in your outbox until you are connected to the Internet again, at which time your messages are sent. Any other actions you take in Gmail will operate the same.
There is also a "flaky connection mode," which still uses your local cache as if you were disconnected, but synchronizes your mail with the server in the background - good when you don't have a strong connection. The offline version of Gmail should be available under your "Labs" tab. If not, it will appear shortly. Below is a video explaining Gmail offline.