FLC OF AN MLG ABORTED TKOF WHEN ANOTHER ACFT WAS OBSERVED XING THE DEP END OF THEIR RWY. TWR HAD NOT CLRED THEM FOR TKOF!

Narrative

FLT FROM ATLANTA TO PHILADELPHIA; OUR ACFT WAS CLRED TO POS AND HOLD RWY 26L AT HARTSFIELD. A PRECEDING ACFT HAD BEEN CLRED TO TKOF ON RWY 26L AS WE WERE APCHING THE HOLDING APRON FOR RWY 26L. DURING THE REMAINDER OF THE TAXI; THE 'BEFORE TKOF' AND 'CAPT'S TKOF BRIEFING' WERE ACCOMPLISHED. THE TWR WAS XMITTING WITH OTHER ACFT BEING CLRED TO LAND ON RWY 26R AND CROSS RWY 26L. EVIDENTLY AFTER TAXIING INTO POS; THE CAPT HANDED ACFT CTL OVER TO ME SINCE IT WAS MY LEG. I ADVANCED THE THROTTLES FOR AN EPR CHK; RELEASED THE BRAKES AND BEGAN THE TKOF ROLL. IMMEDIATELY FOLLOWING THE TKOF ROLL WITH ENGS SPOOLED AND AIRSPD BELOW 80 KTS; THE CAPT COMMANDED 'ABORT' AND RESUMED ACFT CTL. HE HAD SEEN AN ACFT STILL XING THE DEP END OF OUR DEP RWY. BECAUSE OF THE COCKPIT BRIEFINGS BEING CONDUCTED; WE DID NOT HEAR OF THE CLRNC FOR THAT ACFT TO CROSS RWY 26L; AND DUE TO THE UNUSUAL POINT AT WHICH WE WERE GIVEN CLRNC ONTO RWY 26L; WE THOUGHT WE HAD BEEN CLRED FOR TKOF. AFTER THE 'ABORT' AND TAXIING CLR OF THE RWY; WE INFORMED TWR THAT WE BELIEVED WE HAD CLRNC TO TKOF ON RWY 26L. BECAUSE OF THE POINT AT WHICH WE WERE GIVEN CLRNC ONTO THE RWY; AND EXTRA LONG TIME WE HAD TO TAXI INTO POS; WE BELIEVE WE HAD CLRNC TO TKOF ON RWY 26L; CONSIDERING THE PRECEDING ACFT HAD DONE SO. EVIDENTLY; OUR COCKPIT BRIEFINGS PREVENTED US FROM HEARING TWR'S XMISSIONS WITH XING ACFT. WE BELIEVE WE HAD CLRNC TO TKOF; HOWEVER; TWR INFORMED US WE HAD NOT. UPON REFLECTION OF THE EVENTS; WE AGREED.