I initially did an upgrade from 10.5.6 to 10.6.0, but it never ran quite right. My Leopard install was done with the DellEFI method, not the NBM/NBI, and it really gummed things up doing an update.
If it was me, I would follow the Mechdrew guide to prep an 8 Gig USB thumb drive, run NBM and make that an installation drive, use another external drive and Time Machine to back up your current Leopard install, then do a fresh install of SL and import all your programs and user settings using Migration Assistant.
Mini 9 | OS X 10.6.7 | NBI 20100616212351| BIOS 05 | 32 GB Runcore | 2 GIGS RAM
10V | OS X 10.6.7 | NBI 20100616212351 |BIOS 06|160 GB|1 GIG RAM| 6 Cell Dell Battery
Back up Mini 9| OS X 10.6.7| NBI 20100616212351| BIOS 05 | 32 GB SuperTalent | 2 GIGS RAM