The concert business is supposed to be the music industry's one sure thing. But not this summer. The Eagles, Rihanna and Maxwell have canceled tour dates. A wobbling "American Idol" tour has flooded the market with discounted tickets, and the resurrected Lilith Fair tour has called off concerts from Dallas to Salt Lake City. Even teen idols the Jonas Brothers announced this week that they're scrapping some shows. * * * With the continued evaporation of recorded music sales, acts at all levels of the talent pool must lean heavily on their live-performance earnings. That's forcing artists to tour more,...