B737 FAILS TO MAKE CABIN ANNOUNCEMENT PRIOR TO TKOF AT BOS.

Narrative

AT NIGHT; IN LIGHT RAIN WITH REDUCED VISIBILITY; WE WERE OPERATING A DELAYED FLT. AT THE BEGINNING OF THE FLT; I BRIEFED THE FO 'DON'T LET ME RUSH YOU -- IF YOU NEED MORE TIME; LET ME KNOW AND WE'LL TAKE MORE TIME.' FO HAD 70 HRS ON THE AIRPLANE; AND DID A VERY GOOD JOB; BUT WAS SLOW IN CHKLIST ACCOMPLISHMENT. WE WERE EXPECTING DEP ON RWY 4L WITH A FAIRLY LONG WAIT FOR TKOF. AT THE LAST MIN OUR DEP RWY WAS CHANGED TO RWY 9. COMPLETED THE BEFORE TKOF CHKLIST ON THE DEP RWY; AND I DID NOT REALIZE THE CABIN PA ANNOUNCEMENT 'FLT ATTENDANTS BE SEATED FOR DEP' HAD NOT BEEN MADE. WE TOOK OFF BEFORE THE 2 FLT ATTENDANTS IN THE MAIN CABIN WERE SEATED AT THEIR ASSIGNED STATIONS. WHEN THEY REALIZED WE HAD BEGUN OUR TKOF ROLL; THEY IMMEDIATELY SAT DOWN WHERE THEY WERE AT THE TIME. EVEN THOUGH IT WAS NOT MY INTENTION TO CONDUCT A RUSHED OP; I ALLOWED LAST MIN OCCURRENCES TO COMPROMISE SAFETY. A LEAD FLT ATTENDANT WITH ONLY 4 TRIPS TO HER CREDIT; 2 FLT ATTENDANTS ON THEIR FIRST TRIP; AND A LOW TIME FO SHOULD HAVE RAISED A RED FLAG TO ME. WE WERE FORTUNATE NO ONE WAS HURT. THIS EXPERIENCE ONCE AGAIN POINTS TO THE NEED FOR GOOD COM; AND TO THE ABSOLUTE REQUIREMENT FOR TAKING ONE'S TIME AND NOT ALLOWING TIME PRESSURE TO AFFECT THE OP.