While in terminal airspace two air carrier aircraft sequentially followed TCAS Resolution Advisories with respect to a Cirrus aircraft apparently well above his assigned altitude and conflicting with that of the air carriers.

Narrative

While we were level at 5;000 MSL in CAVU conditions; an air carrier flight in front of us was given traffic; a Cirrus; VFR 4;500 MSL 12 o'clock. The air carrier reported traffic alerts and claims Cirrus is not at 4;500 MSL...much closer. Approach gave us the same traffic; 12 o'clock; 4;500 MSL and VFR. We did not have visual contact; but the Cirrus reported us in sight (said he had our lights). As we closed; we got simultaneous 'Traffic' and 'Climb; Climb' RA alerts. Cirrus showed 400 below moving to 300 below. The First Officer who was pilot flying began a climb away from traffic in response to the RA. I advised Approach who replied that the Cirrus 'said he was at 4;500 FT and radar confirms'. We climbed to approximately 5;400 FT and the RA ended as traffic passed below us and slightly right of the nose. I caught a glimpse of the Cirrus passing to the right and he indeed did not look to be anywhere near 500 FT below us (or in this case it should have been 900 FT with our climb). I could see the occupants clearly. I advised Approach we were returning to our assigned altitude and in response got a frequency change and 'that was that'.