B737 DEP FROM PHL RWY 9L EXPERIENCED TCAS RA WITH DEP TFC FROM RWY 8.

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RA AFTER TKOF OUT OF PHL. TWR HAD CLRED US FOR TKOF FROM RWY 9L WHILE SIMULTANEOUSLY DEPARTING A COMMUTER ACFT OFF OF RWY 8; CONDITIONS IFR. WE TOOK OFF AND AT 400 FT SELECTED DEP HDG OF 085 DEGS. AT APPROX 1100 FT; WE RECEIVED A TA WARNING FROM AN ACFT OFF OUR L; WHICH PROGRESSED TO AN RA. I CALLED OUT 'DSND' AS THE PF (MY FO) SIMULTANEOUSLY COMMENCED FOLLOWING THE TCAS GUIDANCE (AS THE PLT MONITORING; I NOTED THE GUIDANCE SHOWING THE GREEN ARC AT 100-200 FPM DSCNT). I REMEMBER LOOKING OUT TO THE L TO TRY AND GAIN A VISUAL; BUT NO JOY AS WE WERE IFR. THE RA WAS MOMENTARY; LASTING 3-4 SECONDS; THEN VANISHED. WE CONTINUED OUR CLBOUT AND TURNED TO A 180 DEG HDG PER DEP CTL AFTER SWITCHING OVER. AFTERWARD; ENRTE TO ATLANTA; MY COPLT AND I REVIEWED POSSIBLE CONTRIBUTING FACTORS: 1) THE PHL 7 DEP PAGE AND THE PHL DETAILED ARPT PAGE PAGE BOTH SHOW THE FOLLOWING: RWY 9L IS SLIGHTLY S OF RWY 8. THE DEP PROC HDG FOR RWY 9L IS 085 DEGS. THE DEP HDG FOR RWY 8 IS 086 DEGS. BOTH OF THESE HDGS ARE CONVERGING. WORSE; THE MAGNETIC ORIENTATION OF RWY 9L IS 087 DEGS; RWY 8 IS 088 DEGS. THIS FURTHER EXACERBATED THE CONVERGENCE. 2) I DON'T KNOW WHY TWR WOULD CLR 2 ACFT FOR TKOF IN IFR CONDITIONS GIVEN THIS CONVERGENCE WITHOUT A HDG TO TURN AWAY. 3) THE CONFLICT OCCURRED (NATURALLY) DURING ONE OF THE BUSIER PHASES OF FLT WHEN WE WERE ACCELERATING; ACCOMPLISHING CONFIGN CHANGES; NAV; ACCOMPLISHING THE AFTER TKOF CHKLIST; AND SWITCHING TO DEP CTL. I FEEL WE HAD A LOT OF FACTORS COMING TOGETHER IN JUST THE WRONG WAY TO SET US UP FOR A PROB. PERHAPS; EXAMPLES LIKE THIS ARE ON THE RISE DUE TO THE INCREASED TEMPO OF FLT OPS IN AN OVERCROWDED AIRSPACE.