According to iPhone Buzz, Google is currently testing a "technical concept" which would allow Android and iPhone users to access their GMail accounts whilst offline, bringing their recently announced feature to mobile devices.
VP of Engineering at Google, Vic Gundotra, showed a demo of an iPhone 3G with this capability at the Mobile World Congress this morning, showing a handset in Airplane Mode accessing Gmail. This new feature uses HTML5's AppCache and Database standards, meaning that it will be able to be used on any device that adheres to these standards. This will save time for developers, as they won't have to develop a separate application for each device that will use this functionality. The feature was also shown on the recently announced HTC Magic, and it will also work on Palm's upcoming Pre.
According to iPhone Buzz, it appears to be release ready and full functional, but Google has made no announcements what-so-ever about its release.
Check out a video of Vic Gundotra showing this off below.