CL65 FLT CREW EXPERIENCED COM CONFUSION; DEPARTED BOS RWY 22R WITHOUT ATC CLRNC AND OBSERVED TFC XING DEP END OF RWY.

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WE WERE CLRED INTO POS AND HOLD ON RWY 22R AT BOS. I HAD AN OPERATING EXPERIENCE FO WITH 21 HRS IN THE ACFT. AS WE WERE DOING THE 'BOXED ITEMS' ON THE BEFORE TKOF CHKLIST; I HEARD THE CTLR SAY SOMETHING; BUT I WAS PREOCCUPIED JUST FOR A SECOND; AND ASKED MY FO IF THE CTLR SAID 'CLRED FOR TKOF' FOR US. HE SAID 'YES; WE'RE GOOD TO GO;' AND SO WE STARTED THE TKOF ROLL. AS WE ROTATED; I SAW A B737 CROSS THE DEP END OF RWY 22R (WE WERE AIRBORNE WITH PROBABLY 4000 FT REMAINING UNTIL THE END). I KNEW SOMEONE HAD MESSED UP; AND WAS HOLDING MY BREATH AS TO WHO. THE CTLR SAID; AS WE WERE CLBING THROUGH 800 FT; 'HEY GUYS; DO YOU HAVE A PEN HANDY?' I WROTE THE NUMBER DOWN AND THEN WORRIED MYSELF SICK FOR THE NEXT 3 HRS 20 MINS. SUPPLEMENTAL INFO FROM ACN 679392: TWR CALLED AND TOLD US TO 'TAXI INTO POS AND HOLD RWY 22R.' I READ BACK THE ENTIRE CLRNC CORRECTLY BACK TO THEM. AS THE CAPT STEERED THE AIRPLANE ON TO THE RWY; I DID MY 'BOXED ITEM;' A COMPLETION OF THE BEFORE TKOF CHKLIST; AND WRONGLY ASSUMED WE WERE CLRED FOR TKOF. THIS ASSUMPTION WAS MOST LIKELY A RESULT OF MY PREVIOUS ASSOCIATION OF BOXED ITEMS WITH A CLR FOR TKOF CLRNC. AS FOR THE DETAILS OF THE VERBAL TRANSACTION BTWN THE CAPT AND MYSELF; I HONESTLY CANNOT REMEMBER THE SPECIFIC DETAILS AND DON'T WISH TO GIVE ANY FALSE INFO ABOUT THE INCIDENT AT HAND. THE RESULT WAS MY HAVING OF THE CTLS AND EXECUTING A TKOF WITHOUT A CLRNC. I BELIEVE THAT THERE WERE SEVERAL CONTRIBUTING FACTORS TO THIS INCIDENT. IN ALL FAIRNESS TO THE CAPT; I SHOULD HAVE BEEN MORE ALERT AND NOT ALLOWED MYSELF TO GET DISTR BY THINGS LESS CRITICAL. MY INEXPERIENCE; LACK OF RECENT REAL WORLD EXPERIENCE; AND THE LEARNING OF THE ACARS ARE ALL KEY FACTORS. AS A WHOLE; THE CAPT AND I SHOULD HAVE HAD BETTER COM IN THIS CRITICAL PHASE OF FLT. ON THE ISSUE OF HUMAN PERFORMANCE; I WILL ONLY SPEAK FOR MYSELF AND SAY THAT I'VE ONLY BEEN HOME TWICE IN THE LAST 3.5 MONTHS. THIS IS NOT THE REASON FOR THE INCIDENT; BUT I DO BELIEVE IT HAS PLAYED A ROLE. I HAVE LEARNED TO BE MORE ALERT AND NOW KNOW MORE THAN EVER BEFORE THAT IF YOU ARE NOT SURE -- ASK!