To enable full Draft 802.11n capabilities, you will need to download AirPort_Extreme_802.11n_Enabler.dmg and replace the Distribution.dist file in pkg contents with macdotnub's file. Look on his site for further instructions.
SigmaTel STAC9200 High Definition Audio Controller works perfectly using the AppleHDA solution. Head on over to this thread and download the latest version of the kext. Many thanks to Taruga for this true breakthrough!
SpeedStep support is included in the latest Darwin 8.9.1 kernel release for 10.4.9, but you will need the Power Management package put together by Prasys, macdotnub and duddie to provide full SpeedStep, Sleep and CPU Scaling; you can download it from macdotnub's site or at irc.osx86.hu in channel #appleacpi 153554b96e