Get a user’s role by user ID

As greenshady says the only gotcha you need to watch out for is that $roles is an array and if you have a user assigned to more than one role you will only get one returned to you.
I am coding a plugin which cannot assume a single role per user.
I recently used the following to address the problem:

function tina_mvc_user_has_role( $roles_to_check=array() ) {

  if( ! $roles_to_check ) return FALSE;

  global $current_user;
  get_currentuserinfo();
  $user_id = intval( $current_user->ID );

  if( ! $user_id ) {
    return FALSE;
  }
  $user = new WP_User( $user_id ); // $user->roles

  return in_array( $roles_to_check, $user->roles, FALSE );

}

Example:
$user_role_exists = tina_mvc_user_has_role( array('Subscriber','MyRole') );

how do I list users and their roles for the guest?

@zeal: roles do not apply to guest users.

Thanks greenshady, this code is very handy.
Also, if you don’t want to hard-code the user’s id, you can use this:

$curauth = $wp_query->get_queried_object();
$user_id = $curauth->ID;
$user = new WP_User( $user_id );
if ( !empty( $user->roles ) && is_array( $user->roles ) ) {
	foreach ( $user->roles as $role )
		echo $role;
}