Index: wp-includes/ms-default-filters.php
===================================================================
--- wp-includes/ms-default-filters.php	(revision 25008)
+++ wp-includes/ms-default-filters.php	(working copy)
@@ -18,7 +18,6 @@
 add_action( 'init', 'maybe_add_existing_user_to_blog' );
 add_action( 'wpmu_new_user', 'newuser_notify_siteadmin' );
 add_action( 'wpmu_activate_user', 'add_new_user_to_blog', 10, 3 );
-add_action( 'sanitize_user', 'strtolower' );
 
 // Blogs
 add_filter( 'wpmu_validate_blog_signup', 'signup_nonce_check' );
Index: wp-includes/ms-functions.php
===================================================================
--- wp-includes/ms-functions.php	(revision 25008)
+++ wp-includes/ms-functions.php	(working copy)
@@ -433,7 +433,7 @@
 	$user_name = preg_replace( '/\s+/', '', sanitize_user( $user_name, true ) );
 
 	if ( $user_name != $orig_username || preg_match( '/[^a-z0-9]/', $user_name ) ) {
-		$errors->add( 'user_name', __( 'Only lowercase letters (a-z) and numbers are allowed.' ) );
+		$errors->add( 'user_name', __( 'Invalid Username. Only Letters and numbers allowed.' ) );
 		$user_name = $orig_username;
 	}