Air Carrier aircraft in IMC on the SFO ILS PRM Runway 28L stopped its descent at 3;000 FT because an unknown aircraft was climbing to 2;300 FT inside the No Transgression Zone and TCAS alerted two TA's.

Narrative

We were being vectored for the SFO ILS PRM Runway 28L approach; and in the vicinity and south of ROKME; we were first cleared to 3;000 FT; and then while descending and just south of HEMAN; we were cleared to 2;000 FT and instructed to intercept the localizer and cleared for the approach. When descending through about 3;200 FT; we received a 'TRAFFIC ALERT' from ATC who re-cleared us to stop our descent at 2;500 FT; as ATC had an unknown transponder indicating 2;000 FT in the 'NO TRANSGRESSION ZONE.' We had the target on TCAS with an indicated climb. I stopped my descent at 3;000 FT. The target was heading east and climbing through about 2;200 FT. Through breaks in the clouds; I acquired the light; single-engine aircraft at my 11:00 o'clock position a couple of hundred yards away. We passed directly overhead the aircraft while it showed 2;300 FT on TCAS. We got two Traffic Advisory (TA) callouts but no Resolution Advisory (RA) calls or indications. We continued our ILS PRM Runway 28L approach without further disruption.