I have just read on the local Amsterdam tv website (AT5) that a defective KLM plane returned to Schiphol. Here's a rough translation:
"On Wednesday alert passengers on a flight to New York noticed a strange vibration on one of the wings. One passenger alerted the P/A's to this, who in turn informed the pilots. The pilots noticed the unusual vibrations too, but cockpit instruments showed nothing amiss. ATC at Schiphol elected to order the plane (which was already over the Atlantic) to return to the Netherlands for inspection."
Unfotunately the article doesn't provide any more details such as aircraft type, etc.