The official Pokémon website has announced the new changes to the Modified format for Organized Play tournaments. Below is the press release information.

In the interest of maintaining a healthy tournament environment, Pokémon Organized Play removes older sets from the Modified format at the beginning of each new tournament season. Each rotation challenges existing players to create new strategies while enabling new players to get into Organized Play with a minimal investment.

Last year we announced that our goal was to have a Modified format that starts with 9 regular sets at the beginning of the season and ends the season with about 13 regular sets. Now that we have achieved that goal, we will continue to rotate sets each year to ensure that our Organized Play environment remains healthy and vital moving forward. In addition, future Pokémon TCG sets will become legal as they are released, per the Pokémon Organized Play Tournament Rules. The HS Trainer Kit & Black StarBlack Star Promo Cards numbered DP22 and higher and Black Star Promo Cards numbered HGSS01 and higher are also allowed.

The Modified format for the 2010-2011 season will consist of the following sets:

Logo	Set Name
	Diamond & Pearl Majestic Dawn
	Diamond & Pearl Legends Awakened
	Diamond & Pearl Stormfront
	Platinum
	Platinum Rising Rivals
	Platinum Supreme Victors
	Platinum Arceus
	Rumble
	HeartGold & SoulSilver
	HS Unleashed

Source: http://www.pokemon.com/us/news/op_format_2010-2010-06-02/