Air carrier on visual approach to IWA experienced TCAS RA with light traffic near FFZ; claiming Class B needs revision to avoid similar events.

Narrative

We were in contact with PHX Approach when he cleared us for a visual approach to Runway 12L to Gateway Airport. We were over Falcon Field in Mesa; AZ; descending to Gateway Airport. He also advised us of traffic at 2-3 o'clock position and altitude which I spotted way below no factor. We continued descending into Gateway and configuring for landing. Then another traffic popped up on TCAS -- he was at about 2 o'clock position within 3 miles distance or closer and 500 FT below. We didn't have a visual on him and his position on TCAS appeared to get closer and the descent to the airport was no longer an option because he was now in the way of the descend profile. The Captain said to go around; and I had a visual on aircraft; not sure if Captain had visual at the same time. We started a climb to avoid conflict and then we had an RA -- everything happened so fast. We climbed and cleared conflict. At some point we switched over to Gateway Tower. They said 'Fly on a west heading' followed by a 'Turn to the airport and cleared to land on Runway 12R.' We landed with no further problems. The area in which this conflict occurred happens to be below Class B airspace in a corridor between Falcon Field; Mesa; AZ; and the Chandler Airport; Chandler; AZ. The airplane was in an area that was allowed but not sure he was talking to any ATC. In this case; nobody was really wrong being where we were. The only way to correct this from happening again is to lower Class B airspace or make a requirement to be in radar contact in this area if possible to avoid conflict with passenger jets arriving to these runways in use Runway 12L/C/R at Gateway Airport (IWA).