The new release appears to invoke tethering options for AT&T in the US. iPhone OS 3 introduced tethering support in software, but AT&T has been among the carriers failing to support the feature.
A new configuration panel appears to indicate that AT&T has worked out its issues related to refusing to support tethering on the iPhone (it supports tethering with other phones, but apparently fears that iPhone users would overwhelm its network) and will have a billing program in place by the time iPhone OS 4 ships.
The new Internet Tethering setup panel directs users to call AT&T or visit its website to setup tethering on their account.
Other new features spotted by beta testers as noted by Gizmodo include the ability to view photo Camera Rolls in landscape, more useful wallpaper images, an option to turn off group MMS messaging, and a generally faster interface throughout, ranging from call dialing to Maps.