Index: wp-admin/includes/user.php
===================================================================
--- wp-admin/includes/user.php	(revision 22236)
+++ wp-admin/includes/user.php	(working copy)
@@ -135,7 +135,7 @@
 		$user->user_pass = $pass1;
 
 	if ( !$update && isset( $_POST['user_login'] ) && !validate_username( $_POST['user_login'] ) )
-		$errors->add( 'user_login', __( '<strong>ERROR</strong>: This username is invalid because it uses illegal characters. Please enter a valid username.' ));
+		$errors->add( 'user_login', __( '<strong>ERROR</strong>: This username is invalid because it uses illegal characters. Please enter a valid username. Usernames may contain only letters (a-z), numbers (0-9), dashes (-), underscores (_), and periods (.).' )); 
 
 	if ( !$update && username_exists( $user->user_login ) )
 		$errors->add( 'user_login', __( '<strong>ERROR</strong>: This username is already registered. Please choose another one.' ));
Index: wp-login.php
===================================================================
--- wp-login.php	(revision 22236)
+++ wp-login.php	(working copy)
@@ -312,7 +312,7 @@
 	if ( $sanitized_user_login == '' ) {
 		$errors->add( 'empty_username', __( '<strong>ERROR</strong>: Please enter a username.' ) );
 	} elseif ( ! validate_username( $user_login ) ) {
-		$errors->add( 'invalid_username', __( '<strong>ERROR</strong>: This username is invalid because it uses illegal characters. Please enter a valid username.' ) );
+		$errors->add( 'invalid_username', __( '<strong>ERROR</strong>: This username is invalid because it uses illegal characters. Please enter a valid username. Usernames may contain only letters (a-z), numbers (0-9), dashes (-), underscores (_), and periods (.).' ) );
 		$sanitized_user_login = '';
 	} elseif ( username_exists( $sanitized_user_login ) ) {
 		$errors->add( 'username_exists', __( '<strong>ERROR</strong>: This username is already registered. Please choose another one.' ) );