I can confirm that SL can access and restore files from a previous TM backup drive that was used for Leopard. I did a clean install of SL on my Mac Pro, and also bought a new external drive to use for SL TM backups. However, I kept my old Leopard external drive.
Holding down option while clicking on the TM icon in the menu bar allows you to Browse other TM disks.
I just now browsed that Leopard TM disk and was able to restore a file from 12/2008 just fine onto my SL machine.
Now, if you did the upgrade and are using the same TM backup.....I believe that should work as well (although I don't have one to test).
-Kevin