SCT Controller described a TCAS RA response between an Air Carrier at 11;000 and a VFR Cessna level at 10;500; the Air Carrier failing to notify ATC of the alert and climbing 2;000 FT.

Narrative

An E135; level enroute at 11;000 FT; was issued traffic '12 o'clock; 10 miles; opposite direction VFR Cessna at one-zero-thousand-five-hundred.' Opposite direction Cessna was told to 'ensure you are level at one-zero-thousand-five-hundred for traffic;' and issued the traffic opposite direction. Both aircraft were looking for each other. When the aircraft were approximately 2 miles apart I observed the E135 climbing out of 12;000 FT. When I queried the pilot and told him that the traffic was not a factor; that they had required separation; he responded 'we are responding to an RA.' The aircraft continued to climb out of my airspace; over 2;000 FT above his assigned altitude; and did not return to his assigned altitude for at least 10 miles. The pilot did not advise prior to or during the RA event until after I queried him. My biggest concern is that the aircraft climbed into an opposite direction IFR route which consistently has heavy jet traffic at 12;000 FT. I've experienced many RAs in my time as a RADAR Controller; but never one in which the aircraft climbed so excessively; especially when it occurred with known traffic; legally separated. This could have become a very serious event. Aircraft responding to RA's need to be diligent in advising ATC ASAP; especially in instances where they are reacting to situations involving legal separation. One would think that if 1;000 FT of separation is enough for level IFR traffic that would more than suffice for IFR/VFR separation. Aircraft should not be allowed to deviate more than necessary.