The Sugimori artwork that you see at the top of the Pokedexes is made public by Game Freak. The sprites and so on are not licensed at all, but are used under a "fair use" policy (similar to images on Wikipedia).
However, you won't be able to make a Pokemon game that is 100% legal as they do not give out licenses or permission to make games. Many games like Pokemon Tower Defense have been shut down by GF/Nintendo. But feel free to play around, unless it gets really popular they probably won't touch you.