An EMB145 at 9;000 FT in Class B airspace had a near miss with an aircraft descending in Class B without a clearance.

Narrative

Level at 9;000; Approach issued a traffic advisory of an aircraft at 12 o'clock; 10 miles; 9;300 FT descending; unverified. Traffic was on our TCAS display and the information matched the advisory from ATC. ATC then gave us a clearance to descend to 7;000. Descent was initiated and almost immediately we got the RA to descend; followed by climb and then descend again. Captain took control of the aircraft when the RA was issued to respond to the threat; at the closest point; the intruding aircraft appeared on our TCAS as a red target with '+00' directly on top of us. I informed ATC that we were responding to an RA. The threat was an intruding aircraft operating in Class B airspace apparently without a clearance because he was not talking to ATC. I do not believe the TCAS should ever be MELed. If our TCAS had been inoperative today; we very likely would have had a mid-air collision and dozens of people would have been killed; on our airplane; as well as on the ground in what is a very heavily populated area. The TCAS performed its function perfectly and I am thankful that we had it today.