A BE1900C ON INITIAL CLB AT 800 FT AGL DECLARED AN EMER AND DIVERTED DUE TO THE FORWARD ENTRANCE DOOR BLOWING OPEN.

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ON MAY/XA/00 I WAS THE FO ON FLT ABC; FLL TO XXX. AFTER OUR 5TH LEG IN ACFT WXZ-A; WE HAD A CHANGE OF ACFT TO XYZ. OUR NEW ACFT HAD 3 MEL'S INCLUDING: FLAPS; L FUEL QUANTITY; AND THE CABIN DOOR WARNING LIGHT. WE PERFORMED ALL THE NORMAL PREFLT CHKS. WE DEPARTED FROM THE GATE AT XA38. WE WERE CLRED FOR TKOF AT XA50 ON RWY 9L. AT AROUND 800 FT I HEARD A SLIGHT EXPLOSION (DECOMPRESSION) AND FOUND THE CABIN DOOR HAD BEEN BLOWN OPEN. THE WX WAS VFR AND WE IMMEDIATELY DECLARED AN EMER AND PROCEEDED BACK TO RWY 9L AND LANDED SAFETY. NO ONE WAS HURT. WE TAXIED BACK TO THE RAMP AREA AT FLL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ACFT WAS A BEECH 1900C AND HAD 3 DEFERRED ITEMS LISTED. THE RPTR SAID 1 DEFERRED ITEM WAS THE FORWARD CABIN DOOR WARNING LIGHT WHICH REQUIRED CHKING PRIOR TO DEP FOR BEING CLOSED AND LOCKED. THE RPTR SAID; 'AS THE FO IT WAS MY DUTY TO ACCOMPLISH THIS CHK.' THE RPTR STATED; 'THE FLT OPS MANUAL HAS PICTURES OF WHAT A LOCKED DOOR SHOULD LOOK LIKE BUT WERE VERY POOR AT BEST AND THE DETAIL POOR.' THE RPTR SAID; 'THE FAA GAVE ME A LETTER STATING THE DOOR WAS NOT CORRECTLY CHKED FOR BEING CLOSED AND LOCKED.'