You need to insure that the php process (web server) is a member of the group for the Drupal files.  You should make sure that the owner of the files is some user other than the php process / web server, because you do **not** want to allow the web server to overwrite the Drupal code files.

To change the group of all of the files in your root:

    sudo chgrp -R webgroup /path/to/drupal

To add a group to the web server user:

    sudo usermod -a -G webgroup www-data


For more advice on correctly setting file permissions for Drupal, see [Securing File Permissions and Ownership][1].


  [1]: http://drupal.org/node/244924