Once Snow Leopard is released, Leopard will no longer be sold. Apple is very strict about only having one OS for sale at a time.
Since they control the release and distribution of not only the OS, but the machines that run it, I have never seen them run a clearance on an OS.
I do know that if you have an issue with any software you bought from Apple, such as it becoming unreadable, or loss of system discs that came with one of their machines, they are very good about replacing the media.
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