Because of the new requirements in 10.6 of how PrefPanes? need to behave and what they need to support about every 3rd party PrefPane? is broken in 10.6/Snow Leopard which basically nukes Perian since you can't install the codecs.
I know 10.6 is still 2 - 3 months away but I'm running the dev builds, thought I'd give you a heads up.
For the people who want to use Perian on 10.6, download and install the latest version of Perian and then open a Terminal and copy paste the following commands:
cd /Library/PreferencePanes/Perian.prefPane/Contents/Resources/Components
cp Perian.zip ~/Downloads
cp CoreAudio/A52Codec.zip ~/Downloads
cp QuickTime/AC3MovieImport.zip ~/Downloads
cd ~/Downloads
unzip AC3MovieImport.zip
unzip A52Codec.zip
unzip Perian.zip
sudo mv Perian.component/ /Library/QuickTime/
sudo mv AC3MovieImport.component/ /Library/QuickTime/
sudo mv A52Codec.component/ /Library/Audio/Plug-Ins/Components/
Restart QuickTime and enjoy!