If you absolutely need to enable the editor in subsites, lookin /wp-admin/menu.php circa Line 159 (specific to WP 3.1). Changing if ( ! is_multisite() ) to if ( true ) will re-enable the Theme Editor in all subsites.
This is a hack only intended for use if you understand the consequences. Users with appropriate privileges in a subsite will be able to modify all themes in the Network. Modifying core files is rarely a good idea as they will not persist when you upgrade.
Ticket opened for enhancement: http://core.trac.ww.wp.xz.cn/ticket/17062
