PLT OF SMA GA ACFT DEVIATED FROM ASSIGNED ALT DURING ENTRY TO THE ARPT TFC PATTERN DUE TO A NMAC EVASIVE ACTION DSCNT.

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IT WAS THE FIRST REALLY CLR DAY IN SEVERAL WKS; AND A SAT AT THAT; SO THE SKIES OF THE AREA WERE VERY BUSY. I HAD CALLED INBOUND TO PAO OVER THE OSI VOR AND WAS TOLD TO HOLD FOR 5 MINS. I WAS THEN CLRED TO PROCEED FOR L TFC RWY 30; THE NORMAL APCH FROM THAT DIRECTION. THERE WAS; NEEDLESS TO SAY; MUCH TFC OPERATING IN THE ATA; INCLUDING THE FAMOUS BLIMP POSITIONING ITSELF OVER STANFORD STADIUM. MY RTE TOOK ME JUST TO THE NW OF THE STADIUM. WHEN I WAS ABOUT 2 MI FROM THE ARPT; AT 1500 FT; ONE OF MY PAX URGENTLY CALLED MY ATTN TO AN SMA ABOUT TO CROSS R TO L RIGHT IN FRONT OF; AND SLIGHTLY ABOVE US. I MADE A QUICK DSNDING TURN TO THE R; AND THE SMA APPEARED TO REACT AT THE SAME TIME. I LANDED WITHOUT FURTHER INCIDENT. NONE OF MY PAX REMEMBERED HEARING THE SMA ON FREQ; NOR DID I. THE LCL CTLR TOLD ME LATER DURING MY VISIT TO THE TWR THAT SHE DID REMEMBER CLRING THE SMA TO TRANSITION AT OR ABOVE 2000 FT. LESSON #1: TFC IS NOT NECESSARILY WHERE THEY ARE CLRED TO BE. THE MORE IMPORTANT LESSON; HOWEVER; IS THAT PLTS ARE ALWAYS RESPONSIBLE TO SEE AND AVOID; EVEN WHEN FLYING IN AIRSPACE SUCH AS AN ATA. THE CTLRS ARE VERY GOOD ABOUT CALLING TFC DURING NORMAL TIMES; BUT AS THINGS GET BUSIER THAN NORMAL; THEIR ATTN SHIFTS PROGRESSIVELY CLOSER TO THE RWY ITSELF. PLTS NEED TO BE EVEN MORE OBSERVANT THAN USUAL DURING PEAK TIMES. THE PRESENCE OF THE BLIMP WAS EVEN MORE UNUSUAL AND CALLED FOR EXTRA CAUTION AS WELL.