Exxon Mobil, which for more than 30 years was identified as the underwriter of some of the classiest shows on public television, most prominently Masterpiece Theater, but which withdrew from such sponsorships in 2004, is returning to PBS -- but on a far more modest scale than previously. The company has agreed to partially underwrite the Nightly Business Report, produced by public TV station WPBT in Miami, and Nova, produced by WGBH Boston. The resumption of Exxon Mobil sponsors hip comes at a time when several other corporations are abandoning public television and reportedly creating major funding challenges for PBS. Only last month it was reported that Archer Daniels Midland had decided to end its 14-year sponsorship of The NewsHour With Jim Lehrer.