AFTER LENGTHY DEP DELAYS AWAITING FINAL WT MANIFEST; A B737 DEPARTS AND LATER RECEIVES A MESSAGE FROM LOAD PLANNING REVISING THEIR 'FINAL' WTS AGAIN; WELL AFTER TKOF.

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PRIOR TO PUSH BACK; CLRNC DELIVERY ISSUED A SLOT TIME OF XA:15Z FOR SEQUENCING TO ORD. WE TAXIED TO RWY 22 AND HELD SHORT. ATC CLRED US FOR TKOF RWY 22. WE HAD NOT RECEIVED OUR FINAL WT AND SO INFORMED ATC. ATC THEN ISSUED HOLD SHORT. THEN ATC ISSUED A NEW SLOT-TIME TO ORD OF XA:30Z. AT XA:16Z; WE RECEIVED FINAL WTS AND WAITED FOR OUR SLOT TIME. APPROX 12 MINUTES LATER; WE WERE RELEASED TO ORD. SHORTLY AFTER GEAR RETRACTION; SELCAL CHIME SOUNDED IN THE COCKPIT AND THE FO SILENCED THE CHIME. AFTER 18000 FT; FO RETRIEVED SELCAL AND INFORMED ME OF THAT OUR ORIGINAL WT MANIFEST FROM LOAD PLANNING WAS WRONG AND THE SELCAL CHIME HAD BEEN REVISED WT MANIFEST FROM CENTRAL LOAD PLANNING. THE ORIGINAL WT MANIFEST SHOWED ZFW 101645 TOG 118345; MAC 23.0; TRIM 4.2; AND PAX 128 CREW 2/3. THE REVISED MANIFEST SHOWED THE TKOF GROSS WT POINT BEING 600 LBS HEAVIER. WHILE THIS DID NOT PRESENT A GREAT DIFFERENCE THIS TIME; HAD IT BEEN A LARGER ERROR; THIS COULD HAVE NEGATIVELY IMPACTED THE SAFETY OF THE FLT. MORE CONCERNING TO ME IS BEING SELCALED DURING A CRITICAL PHASE OF FLT AND THE TIME AND FUEL USED BECAUSE WTS ARE NOT RECEIVED IN A TIMELY MANNER. DISPATCH HAD MONITORED OUR EXCHANGE WITH CENTRAL LOAD PLANNING AND ADVISED US THAT RPTING THIS INCIDENT WOULD BE USEFUL IN DRAWING ATTENTION TO THIS. I FULLY AGREE WITH DISPATCH THAT CENTRAL LOAD PLANNING HAS ISSUES IN THIS AREA.