ABC's top newsman Peter Jennings, 66, has been diagnosed with lung cancer, the network confirmed Tuesday. In an e-mail to employees, Jennings said he will anchor World News Tonight when he feels up to it over the next few months as he undergoes chemotherapy. "There will be good days and bad," he told staffers, "which means some days I may be cranky and some days really cranky." Charles Gibson and Elizabeth Vargas will serve as Jennings' primary substitutes in his absence.