/*
function limit_feed() {
	if (is_feed() && !is_category()) { // only apply it to main feed
		query_posts($query_string . "&cat=-3&showposts=10"); // exclude certain category (-catergory id), set posts limit
	}
}

add_action('rss2_head','limit_feed'); // apply it to rss2, /feed/
add_action('atom_head','limit_feed'); // apply it to atom, /feed/atom/
add_action('rss_head','limit_feed'); // apply it to rss, /feed/rss/
add_action('rdf_header','limit_feed'); // apply it to rdf, /feed/rdf/
*/