MLG CLBING OUT OF PHL HAS A TCASII RA ON AN SMA ON A PHOTO MISSION IN THE TCA.

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CLBING OFF PHL RWY 27L AT HIGH RATE; HAD JUST TURNED TO 255 DEGS DEP HDG AT ABOUT 1500 FT WHEN TCASII SOUNDED 'DECREASE CLB; DECREASE CLB;' THEN 'DSND; DSND!' AS WE WERE CLBING AT ABOUT 3500 FPM AND ALL THIS TOOK PLACE VERY QUICKLY; IT WAS ONLY A FEW SECONDS BEFORE RECOGNITION AND ACTION OF PUSH-OVER BEGAN. AT THE HIGH RATE OF CLB; INERTIA IMPEDED AN IMMEDIATE CLB STOP; AND AS WE WENT THROUGH 0 RATE OF CLB DOWN; THE SMA BECAME VISIBLE AT OUR 3 O'CLOCK. IT WAS SPOTTED BY THE FO; WHO SAID THE SMA WAS GOING IN THE OPPOSITE DIRECTION. I NEVER SAW IT. WHEN WE INFORMED DEP OF THE EVENT; HE DIDN'T SEEM TOO UPSET. THEN SAID ANOTHER SECTOR WAS WORKING THAT ACFT; AND THE SMA HAD US IN SIGHT. THE PRESENCE OF THE SMA WAS NEWS TO US. NEITHER TWR NOR DEP MENTIONED HIM; AND WE WOULD HAVE USED THE NORMAL SEE AND AVOID IF WE SUSPECTED AN ACFT ON OUR DEP PATH. EVIDENTLY; HE WAS ON SOME SORT OF LCL 'PHOTO' MISSION OR SOMETHING. I FEEL THAT THE ATC FOLKS FAILED THEIR DUTIES BY NOT MENTIONING THE SMA. THAT; COMBINED WITH OUR HIGH RATE OF CLB; HIGH DECK ANGLE; AND DOING THE NORMAL AFTER TKOF CHKLIST DUTIES; CONTRIBUTED TO A POTENTIALLY HAZARDOUS SIT WHICH COULD HAVE BEEN AVOIDED IF WE HAD BEEN INFORMED BY PHL DEP OF THE OTHER ACFT. I HAVE NO PROB FLYING NEAR OTHER ACFT IN A TCA; IF I ONLY KNOW OF THEM.