MD80 CREW LEFT THE FUEL XFEED VALVE OPEN ON THE PREFLT OF THE COCKPIT. THEY DID NOT NOTICE THE PROB UNTIL THE R FUEL TANK HAD APPROX 900 LBS.

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INADVERTENT FUEL IMBAL. SHORTLY BEFORE DSCNT WE NOTED R MAIN FUEL QUANTITY INDICATING 900 LBS. SUSPECTING QUANTITY INDICATOR PROB WE SWITCHED TO 'A' CHANNEL. AFTER AN INITIAL; 9999; INDICATION; A CHANNEL INDICATED 800 LBS. WE PULLED THE BOOKS TO RESEARCH AND SHORTLY THEREAFTER FO NOTICED FUEL XFEED LEVER IN THE OPEN POS. WE HAD MADE A GATE CONNECTION ACROSS STL TERMINAL WITH LESS THAN AN HR TO AN ACFT REQUIRING AN INITIAL PREFLT. IT IS PART OF MY FO'S HABIT PATTERN TO CHK THE FUEL XFEED CLOSED; AND I BELIEVED I HAD ESTABLISHED A SIMILAR HABIT OF CHKING IT DURING THE BEFORE START CHKLIST. BOTH OF US RECALLED ME STATING THE RELEASE FUEL AND XFEED CLOSED; WHEN WE CAME TO THAT ITEM ON THE CHKLIST. YET SOMEHOW 2 HIGHLY EXPERIENCED PLTS MISSED IT; AND THEN FAILED TO DETECT THE ERROR; DESPITE NORMAL INFLT TIME/FUEL LOGGING PROCS; UNTIL THE RIGHT QUANTITY INDICATOR DROPPED BELOW THE USUAL 4 DIGITS. AFTER CONFERRING WITH THE MAINT CTLR WE ELECTED TO CONTINUE TO A SCHEDULED LNDG AT FLL. AN UNEVENTFUL FLAPS 28 DEG LNDG WAS ACCOMPLISHED; ACFT CTL WAS GOOD AT ALL SPDS; CONFIGNS USING 4-5 UNITS OF R AILERON/RUDDER TRIM. THE LESSON LEARNED: ATTN TO DETAIL IS ESSENTIAL. YOU CANNOT DOUBLE-CHK YOURSELF ENOUGH EVEN ON THE MOST AFTER ENG SHUTDOWN AT FLL. A CHK OF MAIN TANK FUEL OPERATING NORMALLY.