A B737 flight crew reported taking off with incorrect zero fuel weight figure that resulted in takeoff weight almost 2000 LBS higher than planned.

Narrative

After takeoff; we received an ACARS message from Dispatch climbing out of about 6;000 FT MSL. Dispatch advised us that operations informed them of a corrected zero fuel weight. The new zero fuel weight was 1;956 LBS 'greater' than the zero fuel weight from the load sheet. (We took off almost 2;000 LBS heavier than calculated performance.) During the Captain's review of the load sheet prior to pushback; there were NO anomalies. The load sheet verified 110 passengers; matching the Flight Attendant napkin (stating 110 passengers); and load sheet zero fuel weight was within a few hundred LBS of the Dispatch Release's forecasted zero fuel weight. Fuel was correct within 100 LBS. I could not figure out where the error was and assumed; initially; it may have been baggage or cargo loading. First Officer was pilot flying and after climbing above 10;000 FT MSL; checklist complete and checking in with Center; I advised the First Officer I was going to contact operations and would be off. I contacted operations and they advised me that the Agent made an error in the passenger count. We actually had 119 passengers and not 110 passengers. I contacted the Flight Attendants on the service interphone. I am not sure which Flight Attendant answered and I asked them for a passenger count. The Flight Attendant I spoke with asked if I had received the passenger count 'napkin' and I said yes; but now I would like an 'actual' count They called me back and said we had 120 passengers on board. I advised that the passenger count napkin they gave said 110. What happened on this flight is what happens all too often. Our 'A' Flight Attendant asked the Agent how many passengers we had on board; instead of following proper procedure by an actual passenger count; wrote the number 110 on the napkin; with NO actual passenger seat count; then handed it to the flight deck. I advised the cabin crew of the severe safety implications this event has. I reminded them that the passenger count they make is used by the cockpit crew to check for any errors in the load sheet by operations. I informed them we took off almost 2;000 LBS overweight. It is difficult to control human behavior; but a re-emphasis by Inflight Management to our Flight Attendants of the role and importance the passenger count has to the overall safety of the flight is necessary.