According to an article on Boston.com, composer John Williams will not write the full score for the new Harry Potter film:

The Boston week was a brief holiday from composing, a stopover between London, where he saw a rough cut of ''Harry Potter and the Chamber of Secrets,'' and Los Angeles, where he is now busy mapping out the score. Williams will compose about 40 minutes of the music, including all the new thematic material; William Ross will develop the themes for the full score.