A B737-400 HAS AN ALTDEV DURING DSCNT BECAUSE THE FLC FAILED TO RESET ALTIMETER TO 29 PT 92 IN IAH; TX; AIRSPACE.

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DURING THE LAST 45 MINS OF A 3 HR FLT FROM CLT TO IAH WE HAD TO DO EXTENSIVE DEVS FOR WX AND CONTINUOUS DSCNTS FOR MODERATE TURB. APPROX 90 MI FROM IAH AT FL240 WE RECEIVED THE LATEST ATIS. THE APCH WAS BRIEFED BY ME AND I PUT THE NEW ALTIMETER (29.74 I BELIEVE) IN THE STANDBY ALTIMETER AS WELL AS THE FMC DSCNT PAGE (#1 AND #2 ALTIMETERS REMAINED AT 29.92). ATC REQUESTED US TO DSND TO 10000 FT MSL. DUE TO TURB I BEGAN A 'LEVEL CHANGE' DSCNT WHICH STARTED US DOWN AT 3000-4000 FPM. AT THE SAME TIME THE FO STATED THAT A NEW ATIS HAD COME OUT WITH A DIFFERENT RWY AND THAT THE WX WAS NOW 100 FT OVCST AND 1/4 MI. WE BOTH GOT OUT OUR NEW PLATES AND DISCUSSED THE LOW CEILING; I BRIEFED THE NEW APCH AND TOLD THE FO TO PUT THE NEW APCH IN THE FMC ALONG WITH THE MISSED APCH WHICH HAD TO BE BUILT. THERE WAS SOME CONFUSION ABOUT BUILDING THE MISSED APCH SO I BECAME INVOLVED WITH THE BUILD BY DIRECTING THE FO WHAT TO PUT IN IT. THE ACFT LEVELED AT 10000 FT BUT I DIDN'T NOTICE I STILL HAD 29.92. THE FO HAD PUT THE CORRECT ALTIMETER IN HIS SIDE AT SOME POINT DURING THE DSCNT. HE NOTICED WE WERE LOW AND STATED 'ALTIMETER' AS ATC DIRECTED US TO DSND TO 6000 FT. 1) I BECAME INVOLVED WITH THE FMC AND STOPPED 'FLYING THE ACFT' DURING DSCNT (EVEN THOUGH I BRIEFED THAT IT IS OUR TOP PRIORITY DURING OUR DEP BRIEF). 2) FAILURE TO CALL FOR PRELIMINARY CHKLIST PASSING THROUGH FL180. 3) THE DSCNT TO 10000 FT WAS FASTER THAN NORMAL DUE TO A HIGH RATE OF DSCNT AS WELL AS STARTING OUT LOWER THAN NORMAL. (FL240 INSTEAD OF FL310 TO FL370) THERE IS USUALLY PLENTY OF TIME TO BRIEF THE APCH ; DO THE PRELIMINARY CHKLIST; AND BUILD THE APCH IF NECESSARY. I WOULD GUESS I HAD LESS THAN 1/2 THE NORMAL TIME AND THEREFORE LOST SITUATIONAL AWARENESS. 4) PREOCCUPATION WITH MAKING SURE THE FMC WAS CORRECT EVEN THOUGH IT WASN'T NEEDED FOR AN ILS APCH. 5) FAILURE TO CALL OUT FL180 DURING THE DSCNT WHICH WOULD HAVE TRIGGERED THE NEW ALT SETTING.