I am a huge fan of Gmail, but one of the downfalls up to this point is you could not access email offline. Gmail labs has come to the rescue again as they have started to roll out offline Gmail. You will have to enable this feature when it is available for your account. Gmail is actually using Gears in the background to cache your email locally. When your browser looses connection to the internet it will revert to your cached account instead of showing nothing. The one thing to remember is that this feature is very experimental so don’t be surprised if you run into some issues.
Gmail Official Release
Gmail YouTube Video On Offline Mail