COMMUTER ACR HAS GPWS TERRAIN WARNING WHEN THEY DSND TOO LOW ON VECTOR FOR APCH INTO MDT.

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WAS HANDED OFF FROM ZNY TO HARRISBURG APCH. I; AS CAPT; WAS THE PF. THE FO LEFT THE PRIMARY FREQ TO CALL COMPANY. I REMEMBER 3000 FT WAS SET IN THE ALT ALERTER AS WE WERE BEING VECTORED FOR THE 13 ILS MDT. THE FO RETURNED TO THE PRIMARY FREQ AND WAS GOING TO ASK IF THERE WERE ANY CHANGES; TO WHICH I WAS GOING TO SAY NO; STILL GOING TO 3000 FT; WHEN THE GPWS 'TERRAIN ALERT' SOUNDED; FOLLOWED BY 'TERRAIN PULL UP WARNING.' I DISCONNECTED THE AUTOPLT AND STARTED AN IMMEDIATE CLB. OUR ALT WAS 3200 FT AND LEVELED OFF AT 4200 FT WHEN THE WARNING STOPPED. THE FO TOLD MDT APCH WE GOT A GPWS TERRAIN ALERT. THE CTLR CAME BACK AND SAID THE MVA IN THAT AREA WAS 3400 FT. HE THEN ASKED 'YOU SAY YOU GOT A TCASII ALERT?' THE FO THEN STATED; 'NO SIR; WE GOT A GPWS TERRAIN ALERT; AND WE ARE CLBING.' 'WHAT ALT WOULD YOU LIKE US AT?' THE CTLR REPLIED '4000 FT; THE ALT I GAVE YOU;' TO WHICH THE FO REPLIED; 'NO SIR; WE WERE GIVEN 3000 FT; IT'S SET IN THE ALT ALERTER.' WE CONTINUED FOR VECTORS FOR THE APCH AND LANDED WITHOUT INCIDENT. IF THE ALT GIVEN WAS WRONG; THEN WHY DIDN'T THE CTLR NOTICE AND ASK ABOUT OUR ALT. IT'S ALSO INTERESTING TO NOTE THAT FOR THE FINAL VECTOR FOR THE APCH IS TO MAINTAIN 3000 FT UNTIL ESTABLISHED; THEN CLRED FOR THE APCH. COULD IT BE THE CTLR GOT AHEAD OF HIMSELF ON HIS ALTS? REGARDLESS; THE TECHNOLOGY WORKED AND NO ONE WAS HURT. SUPPLEMENTAL INFO FROM ACN 209810: UPON RETURNING TO COM #1; I ADVISED THE CAPT THAT I WAS BACK MONITORING THE #1 FREQ. AT THE VERY SAME TIME THAT HE ACKNOWLEDGED MY RETURN; I NOTICE THAT WE ARE DSNDING TO 3000 FT MSL AND I REMEMBER THAT THE LAST ALT I RECALL BEING ASSIGNED TO IS 4000 FT (ALT THAT I PERSONALLY SELECTED IN THE ALT SET DISPLAY WINDOW).