Johnny Carson had Bette Midler singing to him during his farewell week on NBC's Tonight show in 1992, but tomorrow, Katie Couric will have Tony Bennett and the entire cast of a Broadway show. "We're going to throw her a nice party on May 31st, and the viewers are invited - they should all participate," Jim Bell, executive producer of Today, said in a telephone interview last week. As she leaves to anchor the CBS Evening News, her 15 years of co-hosting Today will be remembered mostly in videos. "From her colonoscopy to some contentious political interviews and some silly...