http://www.pcworld.com/article/159803/offline_gmail_coming_soon_on_iphone_android.html

"....
[Google VP of engineering Vic] Gundotra showed the audience at the Mobile World Congress in Barcelona, Spain a "technical concept" application of Gmail, which stores on the device it is used on not only a user's data, but also the software itself. The most notable use of this technology is allowing offline access to e-mails in Gmail.
...."

The Gmail Offline Mode will work on every mobile browser that support HTML 5

http://www.youtube.com/watch?v=VmjxmOtNZCk