AN A319 FLT CREW EXCEPTS A DIRECT ROUTING FROM ZAU AND FAILS TO NOTIFY DISPATCH.

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AFTER A LONG 40 MIN DELAY FOR TKOF OUT OF ORD; FINALLY GOT AIRBORNE. AFTER REACHING CRUISE ALT; ATC ASKED IF WE WERE INTERESTED IN SOMETHING DIRECT; OR IF WE WANTED TO REMAIN ON COURSE. WE CHKED THE FLT PLAN; AND THERE WERE NO NOTES OR NOTATIONS STATING THAT THE COURSE WE WERE GIVEN WAS FOR WX; TURB; OR ANYTHING SPECIFIC. WE ACCEPTED A DIRECT SOMEWHERE; AND ATC GAVE US DIRECT FMG; 1300 MI DISTANT! AFTER ANOTHER 2 HRS IN CRUISE; WE RECEIVED A PARTICULARLY NASTY; SARCASTIC NOTE FROM DISPATCH. (THIS WAS THE FIRST AND ONLY COM WE HAD HAD WITH DISPATCH TO THIS POINT.) WITHOUT GOING INTO DETAILS; IT SAID THAT WE SHOULD HAVE INFORMED HER IF WE WERE DEVIATING FROM THE FILED RTE. IT ALSO SAID THAT THE RTE WE WERE ON WAS GOING TO BE MORE 'CRAPPY' (HER WORDS) AND USE LOTS OF FUEL. WE HAD ALREADY USED UP LOTS OF FUEL WAITING FOR TKOF; AND THE RIDES WERE FINE. WE LOOKED IT UP IN THE FOM; AND IT STATES THAT DISPATCH MUST BE INFORMED WHEN THE RTE SIGNIFICANTLY DEVIATES FROM THE FLT PLAN. WE WERE BASICALLY PARALLELING OUR COURSE; BUT S OF IT. IT WOULD BE NICE TO HAVE SIGNIFICANT DEFINED. ARE WE SUPPOSED TO INFORM DISPATCH EVERY TIME WE ACCEPT A DIRECT COURSE? OR STATE SOMEWHERE THAT YOU ACCEPT A DIRECT MORE THAN XX MI OFF COURSE.