Google recently launched a new version of Gmail for mobile for iPhone and Android-powered devices built on HTML5. The HTML5 draft adds a lot of exciting functionality; perhaps the most exciting is adding a way for websites to be launched offline. For web applications running on mobile devices however, being able to launch offline can dramatically improve the web application experience. AppCache support on the iPhone is still under development, but as of firmware 2.2.1, it is usable.

References: Full Article | Apple's MobileSafari documentation | Webkit Source Code