If you want to install it in 10.5.5, you need to change the SystemVersion.plist
sudo vi /System/Library/CoreServices/SystemVersion.plist
and change this
You cannot change the SystemVersion.plist back after installation, as iPhoto 09 and iMovie 09 will check this when launch.
If you want iPhoto 09 to skip the check*, you need to use a hex editor
open and change this file
then find (do multiple finds) and replace 10.5.6 to 10.5.5 and save it
*this trick does not work on iMovie 09