Laura Bush delivered a diss to Sean "P. Diddy" Combs by refusing to appear alongside the hip-hop heavyweight at last night's grand opening of the National Underground Railroad Freedom Center in Cincinnati, sources said. Combs was supposed to join the first lady, actress Angela Bassett, U2 frontman Bono, Ohio Gov. Bob Taft, Sen. Mike DeWine, Black Entertainment Television CEO Bob Johnson and other dignitaries at the ribbon-cutting ceremony, but Bush's office nixed the photo-op with Combs. "Her reps made it very clear to Freedom Center that they would not have Laura Bush appearing in the same photo-op as P. Diddy,"...