CHARTER FLT DEPARTS SVFR UNDER RAPIDLY CHANGING WX SITUATION. FAA SAYS NOT LEGAL.

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ATIS PRIOR TO TAXI WAS '...8 OVCST; 10 MI VISIBILITY.' I REQUESTED AND RECEIVED A SVFR CLRNC. DURING TAXI THE GND CTLR UPDATED THE WX CALLING IT 3 BROKEN; 5 OVCST; 10 MI VISIBILITY. STILL BEING ABOVE SVFR MINS; AND PART 135 MINS AND OUR COMPANY OPERATING SPECS I ELECTED TO GO AND WAS CLRED FOR TKOF. AFTER TKOF I CLBED TO 500-600 FT; LEVELED OFF FOR APPROX 1 MIN; THEN PROCEEDED UP TO 1200 FT. AT NO TIME WAS I IN LESS THAN VFR OR SVFR CONDITIONS UNDER PART 91; 135; OR OUR OPS SPECS. EVIDENTLY; THE FIELD WENT TO A 3 BROKEN CEILING SHORTLY AFTER I LEFT. THE FAA IS ARGUING THAT IT WAS BEFORE I LEFT. I DID NOT RECEIVE THIS WX UPDATE AND KNEW NOTHING ABOUT IT UNTIL COMPANY CALLED APPROX 15 MINS AFTER DEP AND ASKED WHAT WX I HAD WHEN I DEPARTED. TO THE BEST OF MY KNOWLEDGE; I NEVER BROKE VFR MINS DURING THE ENTIRE FLT. THE CAUSE OF THE CONFUSION WAS RAPIDLY CHANGING WX CONDITIONS AND ISOLATED POCKETS OF LOW WX. THE WX RPTS I RECEIVED DIDN'T COME AS FAST AS THE FAA SAYS THEY DID. I GUESS THE MORAL OF THE STORY IS; IF IT'S CLOSE TO MINS; EVEN IF IT'S NOT 'LESS THAN' MINS; YOU BETTER NOT ELECT TO GO' -- ESPECIALLY IF THE FAA IS ANYWHERE CLOSE.