Select system folder, then hit restore
I think if you want to revert to 10.5.6 using time machine, you just enter TM, go back to a date right before you did the update, select the system folder, then hit restore. After that, restart. Since there may also be other files, you might want to select every folder on your disk, and hit restore. It's my understanding that this is how it works.
Could someone who has done something like this confirm the process?