While material improvements in iPhone 1.1.2 software/firmware update have thus far proven elusive, it appears that Apple did make one interesting change with the release: a slight boost in operating clock speed. iPhones running firmware/software version 1.1.1 display a clock speed of 400 Mhz and a bus speed of 100 MHz, whereas iPhones running firmware/software revision display a clock speed of 412 MHz and a bus speed of 103 MHz.
These measurements can be taken with the third-party application "SysInfo" which is available through Installer.app for jailbroken iPhones.
Previous reports held that the iPhone uses an ARM processor capable of running at 620-667Mhz. However, it appears that Apple down-clocked the processor in production units, presumably to conserve battery life. The slight clock-speed increase delivered by the 1.1.2 update shouldn't have any dramatic effect on performance, but may make 1.1.2 phones feel a bit snappier.