While there is no official announcement yet, just like the last two premiers, this piece of information has been placed at the Disneyland.com Calendar section...
So looks like the Movie Premiere for Pirates of the Carribean: At World's End will be held one week prior to the movie's official release (May 25th).
Special Theme Park Hours for May 19, 2007
Due to a private event on Saturday, May 19, 2007, Disneyland® Park will be opening at 6:30 a.m. and closing at 5:00 p.m. Disney's California Adventure® Park will extend its hours that day and will be open from 10:00 a.m. until 11:00 p.m.
And if you are planning, don't forget that Disneyland also closes early (4 PM) on Sunday, April 22nd due to a Private Party.