Liza Minnelli has checked herself into rehab and is making "excellent progress," her representative says. "Minnelli has valiantly battled substance abuse over the years, and whenever she has needed to seek treatment she has done so," Minnelli spokesman Scott Gorenstein said last night. "She is currently making excellent progress at an undisclosed facility." The legendary 69-year-old entertainer, who has struggled with alcohol and drug addictions, underwent back surgery in September to help heal an injury that prevented her from performing most of the year, as well as from attending the funeral of Joan Rivers in New York. (Click to read about another celebrity who's reportedly been to rehab at least four times—secretly.)