B737 CREW; IN DSCNT TO BWI; UNDERSHOT THEIR ASSIGNED ALT; MISSING A XING RESTR.

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WE HAD AN ALTDEV APPROX 25 NM N OF BWI. WE WERE GIVEN A CLRNC DIRECT BWI OUT OF 16000 FT AND A DSCNT TO CROSS 20 MI N OF BWI AT 5000 FT FOLLOWED. WE MISSED THE RESTR. ABOUT THE TIME THAT THE DSCNT CLRNC WAS GIVEN; I (PNF) WAS GETTING ATIS; GATE INFO AND ANSWERING A CALL FROM THE BACK. TO ADD TO THE WORKLOAD; WE WERE MONITORING NUMEROUS TSTMS IN THE AREA. NUMEROUS THINGS CONTRIBUTED TO THE ALTDEV. BWI ATIS IS WEAK; SO RECEPTION OCCURS CLOSE IN WHERE WORKLOADS ARE ALREADY HVY. ANYTIME 1 PLT HAS TO LEAVE THE FREQ; HE OR SHE HAS JUST TAKEN THEMSELVES OUT OF THE LOOP. ADD A 1 1/2 HR DELAY FOR WX AND THE LIST OF FACTORS INCREASES. WHILE THE ULTIMATE RESPONSIBILITY REMAINS IN THE HANDS OF THE CREW; THERE OBVIOUSLY WERE NUMEROUS THINGS THAT AFFECTED COCKPIT PERFORMANCE DURING THIS MISHAP. SUPPLEMENTAL INFO FROM ACN 546072: THE CAPT (PNF) WAS CALLED BY THE FLT ATTENDANTS AND WENT OFF THE ATC FREQ TO ATTEND TO THE CALL. AT THIS TIME; BWI APCH CLRED US TO 'CROSS 20 NM N OF BWI AT AND MAINTAIN 5000 FT.' ABOUT THIS TIME WE STARTED PICKING UP SOME CONTINUOUS LIGHT TURB AND THE CAPT RPTED BACK UP FROM BEING OFF. THE CAPT RPTED THAT HE WOULD BE 'OFF' TO GET US A GATE. AFTER SETTING UP A 1000 FPM DSCNT; I SWITCHED TO THE DSCNT PAGE ON THE CDU. THIS WAS MY FIRST INDICATION THAT SOMETHING WAS WRONG. I IMMEDIATELY DEPLOYED THE SPD BRAKES TO THE FLT DETENT AND INCREASED MY RATE OF DSCNT. I ATTEMPTED TO HOLD 280 KTS KEEPING IN MIND THE NEW SPD BRAKE/AIRSPD LIMITATION FOR THE B737-700. I WASN'T GOING TO MAKE THE 5000 FT RESTR AND NEEDED TO INFORM ATC. BEFORE ANY CALLS COULD BE MADE; BWI APCH ASKED US IF WE WERE GOING TO MAKE THE RESTR. THE CAPT TOLD APCH WE WOULD BE HIGH OR UNABLE TO MAKE 5000 FT. WE WERE APPROX 25 NM OFF OF BWI AT 9500 FT AND 250 KTS. BALTIMORE APCH VECTORED US L FOR 180 DEGS AND THEN ULTIMATELY BACK ON COURSE. THE REMAINDER OF THE FLT WAS UNEVENTFUL.