The 2009-2010 league cycle launches on September 21st, and Trainers everywhere will be grabbing their cards and gathering at their local leagues to have fun!
Starting with the Rival season, players of all ages will be able to enjoy all-new seasonal themes based on the toughest Pokémon Trainers in the Sinnoh region, earn all-new league rewards, and even earn credits toward our revamped Player Rewards program! This cycle also marks the first time that the Pokémon League features prizes specifically for Pokémon video game players!

League Rewards
Each season, players will have a chance to earn two different promotional cards. The cards will be printed in a new foil pattern never before used on any other Pokémon card, and not only do they look amazing, but they are also cards that competitive players will be happy to include in their favorite decks! For example, Rival season will offer players a chance to earn Snorlax and Uxie—both of which are popular and instantly recognizable!

In addition to the promo cards, players who play in multiple league seasons and meet certain goals will also have the opportunity to earn unique league-themed Nintendo DS™ skins! These DS skins are available exclusively through league participation and will be rewarded every other season.

For a complete list of cards you can earn as league rewards, and for more about how you can earn all four of the new league-themed Nintendo DS™ skins, check out the league page in the coming weeks!

Source: http://www.go-pokemon.com/op/leagues/