Developers were told not to install the update on any Mac that uses integrated graphics. These computers include: MacBook Air, MacBook, Mac Mini, and some models of iMac. The build number of 10.5.5 is showing up as "build 9F5," however, these pre-release build numbers have a history of changing before the update is released to the general public.
Developers were asked to evaluate many parts of the OS, specifically: AirPort, FileSync, and the Finder. AppleInsider notes that this update fixes over 82 bugs that were reported in 10.5.4.