Bulbapedia says this:
master_trainer_mike presents the speaker with question #631 from lepi1962:
Which cards will be in Jamboree (vending, WOTC own, or)?
master_trainer_mike says, "Ahhhh grasshopper, wait and see :)"
This is really aggravating because not only is it never going to be released, WotC has never ever told anyone what it was going to be like. No scans, no information, no nothing.
The basis of Jamboree is exactly what its name implied; a fun mix of Pokémon cards. Taking cues from other previously unreleased sets Crosstrainer and the Unnamed Wizards Set, Jamboree was going to have a number of cards Wizards created on their own and other cards never previously released in English from sets like the Vending Machine cards and other promos. Ultimately, Jamboree might have actually brought American and European players up to date with Japanese players, as many Japanese cards have yet to be released in English.
A lot for this set, like promotional material and box and pack designs, were created, finalized and ready to be released. However, Nintendo prevented this set from being released, so it never made it into the hands of players.