A B767-200 AFTER PUSHBACK STARTED TO TAXI BEFORE THE TOW BAR WAS DISCONNECTED.

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WE PUSHED BACK FROM GATE XX. DURING THE PUSH; AS USUAL; I MONITORED RAMP CTL FREQ AS WELL AS RUNNING ALL THE REQUIRED CHKLISTS PERTAINING TO THE ENG START AND AFTER START. DURING THIS PROCESS MY ATTN WAS PRIMARILY DIRECTED INTO THE COCKPIT; WITH OCCASIONAL GLANCES OUTSIDE TO VERIFY OUR PUSH PROGRESS. AFTER BOTH ENGS HAD BEEN STARTED; I NOTED (ALL AFTER START CHKLISTS HAD ALSO BEEN COMPLETED) THAT WE WERE STILL IN THE SAME SPOT THAT THE TUG HAD PUSHED US TO. I CALLED FOR TAXI; WAS CLRED TO SPOT X; VISUALLY AND ORALLY CONFIRMED THAT WE WERE CLR ON THE R SIDE OF THE ACFT. WE BEGAN (SLOWLY) TO TAXI; WHEN WE HEARD A 'THUNK' SOUND AND IMMEDIATELY STOPPED AND SET BRAKES. EVIDENTLY; AFTER THE PUSH; THE TUG CREW (AFTER SAYING GOOD-BYE) HAD PROBS RELEASING THE TOW BAR. CONSEQUENTLY; EVEN AFTER PERFORMING ALL THE REQUIRED CHKLISTS; ENG STARTS AND GND CALLS; THEY WERE STILL THERE; BUT I COULDN'T SEE THEM OVER THE NOSE OF THE ACFT. IT SHOULD BE NOTED THAT DUE TO THE HIGH WORKLOAD DURING MOST PUSHBACKS; IT IS NOT UNUSUAL FOR ME NOT TO SEE THE PUSH CREW GIVE A 'THUMBS UP' OR EVEN FAIL TO SEE THEM PULL AWAY.