Procter & Gamble lost its president today: Susan Arnold, a 29-year veteran who drove the company’s high-margin beauty business to $20 billion in sales and went on to oversee all of P&G’s brands, stepped down one day after her 55th birthday.

“My dad retired at 62,” Arnold said, phoning this afternoon on her way to a Walt Disney board meeting. “Then he got really sick. You know what? I wanted to get out when I was really healthy.”

Such high-level departures are always suspect—particularly these days as everyone is on a short leash. But this exit by Arnold, No. 7 on Fortune‘s 2008 Most Powerful Women list, really seems to be motivated by a personal decision. For years, she’s told friends and colleagues that she would probably leave Procter at age 55. That talk, of course, spoiled her chances of succeeding CEO A.G. Lafley, who has told me that he views her as courageous and unusually innovative. Lafley, who turns 62 this June, is expected to retire before age 65. As we at Fortune have been saying for a while, Lafley’s successor will likely be Bob McDonald, P&G’s chief operating officer.

Arnold, who is on the board of McDonald’s as well as Disney, has been a prime target of recruiters. Now she’ll be in their sights more than ever. Though she’s likely to duck their calls, at least for a while. “I’m going to take some time to decompress,” she says, adding that she’s looking forward to spending more time with her two kids, 16 and 13. Arnold, who is gay, raises the children with her domestic partner. And though she won’t talk about the challenge of being a gay leader in corporate America, it clearly has affected her thinking about her career. Will she eventually go for another corporate position? She insists she’s not sure. “I’m flexible,” she says.

So goes another powerful woman, adding to the so-called “leaky pipeline” problem in the corporate world. To which Arnold remarks, “Sorry about that!” In Fortune‘s Most Powerful Women issue in 2007, I featured six women “One Step Away” from the CEO position. Besides Arnold, what’s happened to the rest of them? Morgan Stanley co-president Zoe Cruz got fired by CEO John Mack late that year and hasn’t landed a new job yet. Bank of America chief Risk officer Amy Brinkley is struggling to shore her company as well as her own legacy. Avon president Liz Smith is on track to succeed CEO Andrea Jung when she retires.

Meanwhile, Schering-Plough EVP Carrie Cox, who heads the global pharmaceutical business, woke up this morning to find her company in a $41 billion merger deal with Merck. Only one powerful women in that “One Step Away” class has moved into a top post: Ellen Kullman, now CEO of DuPont.