Index: wp-includes/capabilities.php
===================================================================
--- wp-includes/capabilities.php	(revision 18504)
+++ wp-includes/capabilities.php	(working copy)
@@ -507,6 +507,12 @@
 	 * @since 3.3.0
 	 */
 	function __set( $key, $value ) {
+		if ( 'id' == $key ) {
+			_deprecated_argument( 'WP_User->id', '2.1', __( 'Use <code>WP_User->ID</code> instead.' ) );
+			$this->ID = $value;
+			return;
+		}
+
 		$this->data->$key = $value;
 	}