CRJ200 First Officer reports conflict with a VFR aircraft at 17;000 FT during the JOHNS 2 arrival to CLT. ATC called traffic and crew leveled and turned to avoid a TCAS RA.

Narrative

On the JOHNS2 arrival into CLT; we were in a descent to cross SHINE at 11;000 FT. Just after passing through 18;000 FT; ATC issued a traffic alert for a VFR aircraft in the direction of our projected flight path. The Controller did not know what type of aircraft; and was not in communication with that aircraft. As soon as ATC advised us of the traffic; it appeared on the TCAS display. It was showing the target approximately 500 FT below our altitude. The pilot flying stopped our descent at 17;500 FT. Simultaneously; the pilot flying began a left turn as ATC was instructing us to turn left for traffic. As the target passed us about a mile or two to our right; it appeared to be a low wing single engine aircraft at approximately 17;000 FT.The VFR aircraft was flying legally in the airspace below the Class A floor; however the VFR aircraft was flying dangerously close to a busy approach corridor. The purpose of this report is to increase the awareness of potential traffic conflicts with VFR aircraft near busy arrival corridors. I am not sure if much can be done about this with the current airspace dimensions and requirements in place. A few suggestions might be to add alert area information on a VFR sectional and or WAC chart; or even go as far as to make permanent restrictions around these busy areas. ATC did an excellent job of notifying us of the oncoming traffic. We did see the traffic on the TCAS a few seconds later; but every second counts at the speeds we fly.