By supporting the DeviceOrientation API, Web developers can now use the accelerometer (and gyroscope) when developing Web applications and games. This opens the door for a much more robust environment on the Web and lets developers experiment freely (and continually) without the need to wait for Apple's App Store approval process.
Developers will also be able to access the new AirPrint feature in iOS 4.2.1, allowing for wireless printing directly from Web pages through the new Safari print dialog.
As a Web developer, what's your favorite feature update in Mobile Safari? Let me know in the comments!