Index: wp-includes/ms-functions.php
===================================================================
--- wp-includes/ms-functions.php	(revision 19879)
+++ wp-includes/ms-functions.php	(working copy)
@@ -561,8 +561,8 @@
 	if ( empty( $blogname ) )
 		$errors->add('blogname', __('Please enter a site name'));
 
-	if ( preg_match( '/[^a-z0-9]+/', $blogname ) )
-		$errors->add('blogname', __('Only lowercase letters and numbers allowed'));
+	if ( preg_match( '/[^a-z0-9\.\-]+/', $blogname ) )
+		$errors->add('blogname', __('Only lowercase letters, numbers, dash and period allowed'));
 
 	if ( in_array( $blogname, $illegal_names ) == true )
 		$errors->add('blogname',  __('That name is not allowed'));