HTC tried to do it with their new ‘push internet' in the upcoming Diamond2 and Pro2, but it's monolithic software devs Google that have cracked it - email that works when you're not online.
Following a demo of its mobile app, yesterday Google released new versions of two of its mobile apps for iPhone and Android. Gmail and Google Calendar are now accessible offline thanks to some nifty use of caching, HTML5 tech, and Google Gears. You'll be able to check out recently viewed messages and calendar appointments so that being out of signal wont' be quite the gut-wrenching experience it usually is.
The user interface has also been updated with a ‘floaty bar' - Google's cute moniker for a toolbar of common menu functions that comes up with you select a message - and basically looks more like desktop Gmail.
Meanwhile, check out this neat video from YouTube.