You should be able to use them to get the Panther update package, and save some bucks on the retail version of Panther. My newest ibook, bought in December, came with the drop-in Panther pack in the box, but that's just because the reseller was nice about it. Otherwise, I would have had to turn them in and spend the $20 to order Panther under the upgrade deal.
If you have a local Apple store, they can tell you what they're worth. Also, look on apple.com to see if they're useful (if there's some kind of Panther upgrade deal for those folks who bought one of the transition computers).
They're by no means "worthless". I used them to order X 10.1 after I bought my Pismo new. And I used them to upgrade to 9.0.4 after I got my Strawberry iMac and blueberry iBook. So they have some value if you act with them at the right time.