This morning I backed up and installed all 4 of the software updates that my MacBook needed. After the successful restart and log in I opened Safari to see what all had changed and is new.
One of the less visible, but really cool new features in the underlying WebKit engine v3.1 is CSS Web Fonts. This is a less important new feature for normal users, as it is essentially invisible to the eye, but it does mean that a neater way of web design is now supported on Safari. This website has some examples of CSS Web Fonts, just scroll down to the red, blue, and green squares, those are the examples (they look bad unless you have Safari v3.1 installed).
Also, the Debug menu has been renamed to Develop and has a checkbox in the Advanced tab of Safari Preferences to enable/ disable it. The menu itself also contains less functions now, now it just contains the useful ones, not the odd and useless ones as well. Two new User Agents were added to Safari in the v3.1 update, these are Mobile Safari 1.1.3 – iPhone and Mobile Safari 1.1.3 – iPod Touch. This hints at the ability to use Safari on a Mac or PC to test webpages as they’d appear on the iPhone or iPod Touch. Apple probably wanted to add it now that the iPhone/ iPod Touch SDK has been released.