Sugarless gum is typically sweetened with sorbitol that although less cavity causing (cariogenicity) still is not a cavity-free sweetener.
Xylitol, a naturally occurring sugar alcohol (polyol), is the only compound of those tested (xylitol, sorbitol, mannitol) which significantly did not affect the growth and acid production of from cavity causing bacteria. More about xylitol, click here.
Xylitol gum, mints, and candy have been available for years, but they are usually harder to find or are available only by mail. Now you can find xylitol chewing gum, Spry, locally at Whole Foods (address) and The Vitamin Shoppe (address, gum).