AN ACR FLC IN A TWIN ENG TURBOJET TOOK OFF FROM A NON TWR ARPT BEFORE RECEIVING AN ARTCC RELEASE.

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DEN ADVISED US HE HAD 1 INBOUND 'EXPECT NO DELAY; HOLD FOR RELEASE.' HE THEN CLRED US TO MTJ; MAINTAIN 17000 FT. THE INBOUND ACR FLT APPEARED ON FINAL ALMOST IMMEDIATELY. ANOTHER ACR WAS READY TO PUSH BACK NEXT TO US. WITH RAMP SPACE LIMITED; WE SWITCHED TO UNICOM TO TAXI TO RWY 35. THIS CLRED THE RAMP AND ALLOWED THE INBOUND ACFT TO TAXI BEHIND US ON THE PARALLEL TXWY FROM RWY 17. WE WATCHED HIM TAXI IN; AND CHKED 'MONTROSE TFC' NO OTHER ACFT ON UNICOM. AT THIS POINT WE FORGOT 'HOLD FOR RELEASE.' WE HAD BEEN TOLD TO 'EXPECT NO DELAY;' THE INBOUND ACFT WAS ON THE RAMP BEHIND US; WE WERE CLRED 'TO MTJ; MAINTAIN 17000 FT;' AND THERE WAS NO OTHER TFC ON UNICOM. WE THOUGHT WE HAD BEEN RELEASED; TOOK OFF; AND CONTACTED ZDV AT APPROX 2000 FT AGL. ZDV THEN ADVISED US THAT HE HAD BEEN UNABLE TO REACH US TO ISSUE OUR IFR RELEASE. WE REALIZED THEN THAT WE HAD INADVERTENTLY TAKEN OFF VFR. ZDV STERNLY DIRECTED US TO INSURE WE ARE PROPERLY RELEASED IN THE FUTURE. WE APOLOGIZED AND HE CLRED US DIRECT POWDR. IN RETROSPECT; OUR ERROR WAS OBVIOUS. THE RARITY OF FLTS INTO UNCTLED ARPTS ACCLIMATES US TO THE TEMPO AT LARGE; TIGHTLY CTLED ARPTS. I WILL BE MORE ALERT IN THE FUTURE. ANOTHER CONTRIBUTING FACTOR WAS OUR COM SETUP. WE HAD ZDV AND UNICOM BOTH ON VHF #1; AND TOGGLED BTWN THEM. BETTER TO HAVE PUT THEM ON SEPARATE RADIOS TO MONITOR ZDV WHILE TALKING ON UNICOM.