Last week, Pokemon.com announced that the Modified format for next season will be Majestic Dawn-on. The following sets will be rotated as of September 1st, 2010 and will no longer be allowed for tournament play:

Diamond & Pearl
Mysterious Treasures
Secret Wonders
Great Encounters
DP Trainer Kit
POP 6-9
Promos DP01-DP21

Remember that the Modified format is for official premier events like Battle Roads and City Championships. Casual play and many Leagues may still allow rotated sets to be played.

Note: You can still use cards from rotated sets if that card has been reprinted in a legal set.
Example: You can use Rare Candy from Great Encounters in a Modified deck because it was reprinted in HS-Unleashed.