I got this off Mugglenet, just a second

Afraid of alienating die-hard Muggles by cutting scenes, the studio and scribe initially had talked about breaking the book into two separate films to be released in the same calendar year, à la The Matrix Reloaded and Revolutions, but Warners has now opted for a single movie.

A rep for Warners said the studio will likely bring back the franchise's aging kid stars for the fourth adventure, but it's possible filmmakers may decide to recast. A separate announcement regarding those roles will be made soon.

A 750+ page book into a 2 and a 1/2 hour movie? Steve Kloves is going to have to make some difficult decisions. Many cuts will have to be made.