get_users_of_blog() changed in 3.1

up until now this would work in my plugin:

$blogusers = get_users_of_blog();
foreach( $blogusers as $user ) {
  if $user->has_cap( 'whatever' )
    do_something();
}

Are the items returned not class WP_User anymore?
Is it a bug or was I just lucky it did work before?