Vic Gundotra, VP of engineering at Google, demonstrated an offline-capable webapp for the iPhone 3G this morning that allows full access to a user's GMail even while the handset is in airplane mode.  The app - which Gundotra described as a "technical concept" - relies on HTML5's AppCache and Database standards, keeping an on-device store of not only the user-data but the software itself.

Because it uses HTML5 standards, rather than something unique to the iPhone 3G, it means that the software can be run on any other device which adheres to those standards.  That enables developers to code a single app which will run on multiple platforms, with the GUI and functionality identical on each; Gundotra demonstrated the GMail app running both on the iPhone 3G and on the recently-announced HTC Magic, which uses the Android OS, since both use the Webkit browser engine.  That same engine is also used by Palm for their Pre smartphone, and as such the app will run on that handset too.

As well as offline support, there's also a floating toolbar that tracks the user's email browsing, together with support for message labeling.  However despite appearing release-ready in the demonstration, Google have made no announcement regarding when the offline-capable GMail app might be released.