Just noticed this in the source code:
header( "WP-Super-Cache: Served supercache file from PHP" );
I'm using the mod_rewrite method, but others might not. Announcements in the headers that are sent may be okay with some people, not okay with others. More importantly though, custom headers that are non-RFC should be prefixed with 'X-' such as 'X-WP-Super-Cache'.