B737 EXPERIENCED GPWS AT 5000 FT NEAR MOUNT DIABLO WHILE ON VECTORS TO OAK.

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ON ARR INTO OAK OFF THE MANTECA 1 STAR; AFTER LOCKE AND ON A 260 DEG VECTOR; WE WERE LEVELING AT 5000 FT (IN IMC; 250 KTS AND CLEAN) WHEN WE GOT A 'TERRAIN; TERRAIN; PULL UP; PULL UP' ALERT (PROBABLY GPWS MODE 2). WE EXECUTED THE ESCAPE MANEUVER AND THE 2 PULL-UPS IMMEDIATELY CEASED AS WE BEGAN TO CLB. THE CAPT RETARDED THE THROTTLES THEN FROM MAX TO TKOF THRUST AND THEN TO IDLE AS I NOTIFIED ATC WE WERE RESPONDING TO AND CORRECTING BACK TO 5000 FT FROM A TERRAIN ALERT. THE MAX ALT OBTAINED WAS AROUND 6000 FT AND THE MAX N1 AROUND 83% AS THE ENGS DIDN'T HAVE TIME TO SPOOL ALL THE WAY TO MAX BEFORE THE MANEUVER WAS COMPLETED AND THE THROTTLES RETARDED. FROM THE FIRST 'TERRAIN' ALERT; I IMMEDIATELY LOOKED AT THE EGPWS DISPLAY AND NOTICED THERE WAS ONLY A SOLID GREEN SQUARE AREA BENEATH THE ACFT. ATC RESPONDED WITH SOMETHING LIKE 'UHHH; YEAH; YOU CAN GET THOSE OFF OF MOUNT DIABLO....' HE THEN STATED THAT MINIMUM VECTORING IN THAT AREA WAS 4000 FT; AND LATER CORRECTED IT TO 4900 FT. MOUNT DIABLO IS 3842 FT THOUGH IT APPEARS THERE MAY BE AN ANTENNA ON IT RAISING IT TO 3899 FT. THE COMMERCIAL CHART TALKS ABOUT GPWS ALERTS SE OF THE ARPT; BUT THIS WAS NE OF THE ARPT ON VECTORS FOR RWY 11. AS DIABLO IS THE HIGHEST TERRAIN WITHIN 25 MI OF OAK; PERHAPS THERE ARE GNDS FOR NOT DSNDING PEOPLE RIGHT ON TOP OF IT IN IMC TO BASICALLY THE MVA; PARTICULARLY AS IT APPEARS IT ISN'T UNUSUAL TO GET AN ALERT; ACCORDING TO ATC. THIS EVENT ALSO DRILLS HOME THE IMPORTANCE OF KNOWING THE HIGH TERRAIN ON APCHS AND HAVING GOOD POSITIONAL AWARENESS AT ALL TIMES; EVEN THOUGH WE KNEW WE WERE ABOVE THE MSA.