I think the HTTPS element will only be present under Apache 2.x. It's not in the list of "special" variables here:
http://httpd.apache.org/docs/1.3/mod/mod_rewrite.html#RewriteCond
But it is here:
http://httpd.apache.org/docs/2.0/mod/mod_rewrite.html#rewritecond