Index: wp-includes/ms-functions.php
===================================================================
--- wp-includes/ms-functions.php	(revision 14920)
+++ wp-includes/ms-functions.php	(working copy)
@@ -556,6 +556,11 @@
 		add_site_option( 'illegal_names', $illegal_names );
 	}
 
+	// On sub dir installs, Some names are so illegal, only a filter can spring them from jail
+	if (! is_subdomain_install() )
+		$illegal_names = array_merge($illegal_names, apply_filters( 'subdirectory_reserved_names', array( 'page', 'comments', 'blog', 'files', 'feed' ) ) );
+
+
 	if ( empty( $blogname ) )
 		$errors->add('blogname', __('Please enter a site name'));
 
Index: wp-admin/ms-edit.php
===================================================================
--- wp-admin/ms-edit.php	(revision 14920)
+++ wp-admin/ms-edit.php	(working copy)
@@ -148,6 +148,12 @@
 		$domain = '';
 		if ( ! preg_match( '/(--)/', $blog['domain'] ) && preg_match( '|^([a-zA-Z0-9-])+$|', $blog['domain'] ) )
 			$domain = strtolower( $blog['domain'] );
+
+		// If not a subdomain install, make sure the domain isn't a reserved word
+		if (! is_subdomain_install() &&	in_array($domain, apply_filters( 'subdirectory_reserved_names', array( 'page', 'comments', 'blog', 'files', 'feed' ) ) )
+		)
+			wp_die( __('The following words are reserved for use by WordPress functions and can not be used as blog names: blog, comments, page, files and feed' ) );
+
 		$email = sanitize_email( $blog['email'] );
 		$title = $blog['title'];