~Below I've just written up some instructions for you faint of heart about the Leopard upgrade. Bookmark this post if you need them.~
To the people worried about reformatting (and in simple terms, yes you will need to if you want ZFS), here's what you'll want to do:
0. Delete all the particularly "hacky" apps you use. Remove APE, Little Snitch, or just about any program that launches a daemon (GimmeSomeTunes, Quicken, etc.). The reason for this is that since you're doing an Archive & Install of Leopard, and since these apps will auto-launch at boot or login, if they're incompatible with Leopard they might bork your startup sequence and make Leopard unbootable without troubleshooting.
1. Buy Leopard
2. Run the Disk Utility on the Leopard DVD and create "New Image" from your current system disk to back it up to an external drive; generally just pick compressed and be on your way (Mac lingo for this is to 'create a sparse disk image').
3. After you've made the image, opened it and let it verify to make sure it was taken properly and isn't corrupt, THEN you'll use Disk Utility to "Erase" the main system drive with the partition options... dialog set to "GUID partition map" and the file format to ZFS. Erasure will probably only take a minute or two.
4. Still selecting the system disk, you'll click the Restore tab and drag the backup image you just made onto the image field. Click restore to restore that image onto the system disk (now with fancy ZFS!). The irony at this point is that since Tiger doesn't (and probably won't) support ZFS, the disk probably isn't even bootable! However, the Leopard installer probably won't know or care and will recognize it as a valid upgradeable install.
5. Now you'll quit Disk Utility and go through the OS X installer, selecting "Archive & Install" as opposed to "Erase and Install" or whatever they'll be calling it. (Be sure to go into options and flag as much as possible to be preserved, this will leave your system almost entirely as you found it before upgrading.)
6. After Archive & Install is complete, your user directory and apps (etc) should be in their proper places, with your old remaining/system files in '/Previous Systems/Previous System 1'. Enjoy Leopard & ZFS without having to dump any of your data/apps![/code]