The Official Gmail Blog has announced the availability of Offline Gmail. Check out their post and the intro video.
The Google Operating System blog also has a thorough overview of Offline Gmail and they also point out a couple of downsides such as no offline contact access for now and no attachments when composing a message.
Overall this is a handy tool for folks who want to compose an email or re-read a recent message while away from an internet connection and don’t want the hassle of trying to keep their messages archived in a local email client. This should help remove one of the obstacles it seems many people have when I point them to Gmail — they often ask, “How do I get my email offline?”