For what it's worth; In 2006 and 2007 I always got SSSS on my boarding pass when traveling in or to the US. With roughly 10 trips a year from Europe to the US it was obviously quite annoying with all the extra security controls, but only once did it cause real troubles; In Charlotte I was denied boarding on a flight to Frankfurt. I newer got a good explanation about why, but later got an apology from LH and they covered all my expenses for accommodation and new ticket.