Update Monday, October 29 at 11:30am:
As it turns out, I went ahead and installed Leopard anyway. I figured, "Why not? I've got everything backed up." So I ran the installer but did an archive and install, where Leopard is installed in a new System folder.
Everything is running fine, and a day later, now my disk's S.M.A.R.T. status says it's okay! So perhaps Leopard does something odd. I'll be watching this closely for the next week.
Last night, I went to my local Apple store, hungry for the chance to buy my copy of Leopard. I had pined for this day for months. And there I was, in the line, at the front of the line, laying down my money, and finally--finally!--picking up the shiny holographic box with my tiny little hands.
It's mine! MINE! BWAHHAHAHAH!
I drove home in a frenzy, and walked into my house.
And sanity hit me up beside the head with a brick.
I breathed deeply, and realized: if I am going to install Leopard, I have to do this right. Which means: you back up first.
The imp inside me said, No! You can't wait! You have to start immediately! You have to be the first to install!
After beating my internal imp into submission, I dutifully started running my backup program. And I did my household chores while I waited for the backup to run. I did dishes. I fed the cat. I cleaned her water bowl. I put clothes away, washed my face, brushed my teeth--all the good stuff.
Finally, I checked the computer and the backup was done. FINALLY! said the bludgeoned imp inside me. So I stuck the Leopard DVD in and rebooted.
And again, sanity hit me up beside the head, with the same brick. So I started to run the Disk Utility on the Leopard DVD. By this point, the imp inside me was groaning and clawing (but gingerly, as it was still a bit bruised) and realizing that it wasn't going to win.
And that's when the bad news hit.
There it was. Disk Utility said that the S.M.A.R.T. disk in my machine had determined that it was failing. My imp shouted, But I've only had this machine for a year! It can't be failing!
And somewhere deep in my brain this brain cell fired up--a brain cell storing information regarding some potential issues with this particular model of Macs having problems with hard drives failing. Oh, I can't find a link now that confirms this memory, but it's moot.
So now I have to wait until Monday to take my laptop in to the Apple store (it's covered under an AppleCare agreement) and talk to the bods about it.
Forgive me while I wear black, but I am in mourning.