Publishers Weekly reports that Dan Clancy, who has run the Google Books program and been its tireless public face, will be working for YouTube. My understanding is that Clancy will remain involved with the policy and legal aspects of Google Books.
UPDATE: Via ResourceShelf, the official Google statement:
Google engineering director Dan Clancy is taking responsibility for YouTube engineering. In the coming months, he will continue to be involved in key technology and business issues for Google Books. Engineering director James Crawford will continue to oversee engineering for Google Editions, the Google Books Library Project and the Google Books Partner Program.