_cron() only updates feeds wich have been updated before

I imported a lot of feeds on my site today, and I noticed that the feeds are not automatically imported when _cron() is run. This is 'cause the function looks like this :
function import_cron() {
$result = db_query("SELECT * FROM feed WHERE timestamp + refresh < ". time());
while ($feed = db_fetch_array($result)) {
import_refresh($feed);
}
}

when it should look like this :
function import_cron() {
$result = db_query("SELECT * FROM feed WHERE (timestamp + refresh < ". time() .") or (timestamp IS NULL)");
while ($feed = db_fetch_array($result)) {
import_refresh($feed);
}
}

It's not that important with a few feeds, but with larger (I got about 80 now), it gets tiresome to click 'update items' for every feed ;-)