Gah it was 2005 when this book came out? Doesn't seem that long ago but I guess it is.
Anywho, the movie is actually gonna be released in two parts, so we're gonna have Harry Potter and the Half-Blood Prince 1 and Harry Potter and the Half-Blood Prince 2. They say it's because it would be too long otherwise but given the fact that they managed with the 5th book, and the precedent set by Return of the King, I'm thinking it's just because they want us to have to pay for 2 different movies.