Besides supporting the latest iPhone OS version, beta 5 doesn't do a whole lot else besides throwing in small tweaks to the UI, tweaks to the developers tools and some bug fixes. No big feature addition like last time, but you gotta update if you're making iPhone apps. You just gotta! There probably won't be very many more of these before the next iPhone's out.
This ﬁfth beta release of the iPhone SDK includes a complete set of tools, compilers, frameworks, and documentation for creating iPhone OS applications. These tools include the Xcode IDE, and the Instruments analysis tool, among many others. With this software you can develop applications that run on iPhone and iPod touch using the included iPhone Simulator.