A321 CREW HAD #2 ENG COWL DEPART THE ENG AND IMPACT THE WING LEADING EDGE AND SLAT SYS.

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AFTER TKOF FROM PHL WE NOTICED A VIBRATION ON CLBOUT. THE FLT ATTENDANT CALLED THE COCKPIT AND SAID THERE LOOKED LIKE SOMETHING WAS STICKING UPON THE R WING. WE CHKED ALL COCKPIT INDICATIONS WHICH WERE NORMAL AND THE VIBRATION WAS JUST ABOUT GONE. THE FO WENT BACK TO CHK OUT THE SIT AND RETURNED TO SAY THE COWLING HAD COME OFF THE R ENG AND LOOKED LIKE IT WAS UP AGAINST THE LEADING EDGE OF THE WING. WE NOTIFIED ZDC WE NEEDED TO RETURN TO PHL. WE WERE SOMEWHERE ABOUT 20000 FT AND NEEDED TIME TO DSND AND PREPARE FOR A NO FLAP LNDG IN PHL BECAUSE AFTER TALKING IT OVER WITH THE FO AND ANOTHER COMPANY PLT IN THE CABIN WE DID NOT WANT TO PUT OUT ANY LEADING EDGE DEVICES WITH THE COWLING OVER THE WING. WE CAME BACK TO PHL AND MADE AN UNEVENTFUL APCH AND LNDG. AN EMER WAS DECLARED BY PHL BECAUSE WE REQUESTED THE TRUCKS TO STAND-BY. AS FAR AS A RECURRENCE OF THIS SIT WE NEED TO FIND OUT MORE INFO FROM AIRBUS I UNDERSTAND AT THIS POINT THERE HAS BEEN SEVERAL COWLING PROBS AND WILL FIND OUT MORE INFO.