RETURN LAND MANDATED BY FO'S AIRSPD INDICATOR BEING UNRELIABLE WITH FO'S AIR DATA COMPUTER BEING INVALID.

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DEPARTED MIA RWY 9L. AS PNF; I CALLED V SPDS FOR TKOF USING MY ASI (AIRSPD INDICATOR). FO ROTATED SLOWLY; BUT I DIDN'T FIND OUT WHY UNTIL LATER. SHORTLY AFTER DEP; OUR AIR DATA COMPUTER; WHICH SUPPLIES AIRSPD; ALT; AND RATE OF VERT SPD INFO TO EACH PLT (2 COMPUTERS); PUT OUT A MESSAGE SAYING IN EFFECT THAT THE FO'S COMPUTER DATA WAS INVALID. THE SAME MESSAGE WAS RECEIVED WHEN WE SWITCHED TO THE CAPT'S SIDE. UPON EXAMINATION OF THE CHKLIST; WE DETERMINED THAT WE SHOULD RETURN TO KMIA. AT THIS TIME; WHEN ATC GAVE US AN AIRSPD LIMIT (170 KTS). WE REALIZED THAT THERE WAS A 20 KT DIFFERENCE BTWN AIRSPD INDICATORS (FO'S SLOWER). AFTER LNDG; THE AIR DATA COMPUTER ACTED NORMALLY -- NO INVALID MESSAGES. MAINT WAS NOTIFIED. UPON FURTHER DISCUSSION AND EXAMINATION. MAINT FOUND WHAT APPEARED TO BE FLUFF STUCK ON THE FO PITOT TUBE; PARTIALLY OBSTRUCTING IT. LATER; ONE OF THE RAMP PERSONS SHOWED ME A LARGE SCRAPE ON HIS ARM. THE RAMP PERSON WAS BUSY EMPTYING THE LAVATORY DURING OUR PREFLT OF THE ACFT AND EVIDENTLY HIT HIS ARM ON THE PITOT TUBE; AND THAT WAS WHAT THE 'FLUFF' WAS: HIS SKIN AND HAIR HELD ONTO THE TUBE BY HIS QUICKLY CONGEALING BLOOD. LESSON: RAMP FOLKS SHOULD BE INFORMED THAT IF THEY CUT THEMSELVES ON ACFT EQUIP; THEY SHOULD TELL EITHER MAINT OR THE CREW (WE HAD ALREADY COMPLETED OUR PREFLT WHEN THIS OCCURRED).