iOS 4.3 introduces the personal hotspot feature, first seen in the iPhone 4 on Verizon. This allows the iPhone to create a virtual hotspot for use by up to five devices; three over WiFi, three over bluetooth and one over USB. Data access over the virtual hotspot may be routed through a different APN (the data routing access point for the network) and charged under different rates to the main iPhone data plan.
Another addition is that of iTunes home sharing, which brings iTunes into the world of media centres. Although it's always been possible to stream media from an iTunes server to an iTunes desktop computer, this integrates other iOS devices to consume (as well as broadcast) media to other AirPlay devices.
For iPad users, iOS 4.3 brings sanity back to the hardware switch, as a software setting which allows changing between mute and orientation lock. For both iPad and iPhone/iPod devices, the new operating system brings improved performance, particularly in the WebKit implementation of the Safari browser
Although the iOS 4.3 announcement specifically excludes the iPhone 4 CDMA model, it is likely that the initial OS version is very similar to iOS 4.3, and will be revved if needed with a point update later.
Finally, Apple has released of Xcode 4 (release notes). This is a significant reworking of the underlying build too, similar to the evolution of Xcode replacing Project Builder in the early stages of OSX development. It includes Git as standard, and uses the LLVM toolchain (version 2.0), as well as the new LLDB debugger. Although Xcode 3 remains available for free, Xcode 4 is only available to those with a paid developer account; however, Xcode is also available as a $5 download via the Mac App Store for non-developers. This is the first time that an Apple development kit has not been available for free since OSX was released; whilst it won't impact any current developers it may dissuade those from initially trying out development on the OSX platform.
Additionally, Xcode 4 now integrates functionality from Interface Builder, which is used to create UIs for OSX and iOS systems. Rather than having two separate applications, Xcode 4 now allows the UI to be edited in-place, in a similar way to editing source files. IBPlugins are deprecated, so that they are no longer available in Xcode 4, though systems will continue to build if they have generated UIs in the past with IBPlugins.
Note that not only does Xcode 4 require an Intel Mac (10.6.6 or above), it also removes the availability for developing PPC applications or kernel extensions.