MacFixit is reporting that you can use Time Machine to backup your system to an external drive before you install Leopard. It's apparently an option within Disk Utility on the install disk.
Except System Restore never really worked, and it only backs up your status. Time Machine is a lot more like Retrospect. Considering you're getting that for 'free' it's not a bad feature at all. Aside from that, what RacerX said.
Did I not hear things right, or did the person presenting Time Machine say something along the lines of, "So as seen as you start to edit any document... Time Machine will make a backup copy of that document..."
The Time Machine FAQ should address most of these issues, so, to consolidate discussion, I'll close this thread and ask people to move to that thread. I won't merge threads because I think it'd just be confusing.
If you feel there's a compelling reason to re-open this, please PM me.