South Korean sources claim Blizzard is going to officially announce Starcraft 2 on May 19th at the World Wide Invitational:
According to a post at Thisisgame.com, Blizzard will announce the sequel for StarCraft in the home land of StarCraft: South Korea. The World Wide Invitational (WWI) is a Blizzard event where top gamers are invited to play Blizzard's games, and that's where the announcement is claimed to be.
Blizzard's strategy for announcing new games have traditionally been by surprising the crowd at one of the major game events. According to Thisisgame, over 100 international media partners have been invited to the World Wide Invitational, and to the possible announcement of a sequel.
According to rumours Starcraft 2 uses a 3D engine and it's also said that a new race will be introduced in this game.