ZJX controller described loss of separation at approximately 12;000 FT; when air carrier failed to comply with altitude restriction; conflicting with another aircraft.

Narrative

I was training Mr. X at R57/R58 St. Augustine sectors. At the time we had 6 arrivals with minimum spacing that we had to get 5 miles and increasing. After giving the restrictions to all aircraft; Aircraft X was observed south of Ormond Beach (OMN) not decreasing his speed and being high for the restriction at LAMMA. Aircraft X was given a clearance to reduce speed to 250 KTS and the next clearance was to stop at 13;000 FT. Aircraft X was at 14;000 FT when the clearance was given and read back. The next 2 radar updates were 13;800 FT and 13;100 FT. The 4th update was 12;600 FT; at this time the pilot of Aircraft X advised ATC he was returning to 13;000 FT. At no time did Aircraft X advise ATC he could not accept or comply with the restriction. Supplemental information from ACN 815923: Clearance from ATC: 'descend and cross LAMMA 250 KTS; 12;000 FT.' During the descent at about 14;000 FT; ATC commanded: 'Stop descent at 13;000 FT.' Captain set 13;000 FT in the Altitude Window and I called and pointed 13;000 FT. Then I was busy with my other Pilot Not Flying duties (radio dial; approach plate verification) when I heard the autopilot disconnect beep sound and I looked to the altimeter indicating 12;750 FT and I saw the Captain hand flying and climbing back to 13;000 FT. I asked the Captain about busting an altitude and he reassured me that he didn't.