Joan Smith: What is Posh's special talent? Being married

Her entire career now consists of being Mrs Beckham

A few years ago, someone took a photograph of the Beckhams on a shopping trip in which the England player appeared to be shielding his wife and one of their children from imminent attack, like an aid worker hurrying refugees from a war zone. Last week, Mrs Beckham was snapped shopping on her own, or as on her own as very famous people ever get, which is to say accompanied by a chauffeur-driven car. For this expedition, on a freezing cold December day, she turned up without a coat, wearing a polo-necked jumper so tight it was impossible to avoid staring at her breasts. (Not, I imagine, that anyone really tried.)

Her picture duly appeared everywhere, with sniggering comments on the admittedly rather curious shape she currently displays. This follows wall-to-wall coverage of her new haircut - the result, apparently, of ditching her hair extensions - and her book of fashion tips, which I see all over the place despite the fact that it's been much derided. As someone who has had more than enough of Diana, Princess of Wales - not engaged, not pregnant, not murdered and not very interesting - I am becoming increasingly irritated by Mrs Beckham, whom I have never even met, because of the huge amount of space she occupies in public life.

There are other women who fascinate the media almost as much but they tend to have jobs. There are various reasons why commentators might dislike Kate Moss (not least her boyfriend, who could give any woman a run in the narcissism stakes) but she and Britney Spears do more than have their pictures taken for celebrity magazines.

After a brief and undistinguished career in a girl band, Mrs Beckham married a famous footballer. That's about it as far as her career is concerned, and she might easily have disappeared into that shadowy half-world of former celebrities - another former Spice Girl, Melanie Brown, resurfaced recently when an ageing American film star questioned the paternity of her unborn child - had she not turned herself very skilfully into a brand.

Princess Diana was a brand, perhaps the first in the modern era. Being a brand is a full-time occupation, requiring not just endless changes of wardrobe - by the time Mrs Beckham went for a drink at Claridge's on the evening of her shopping trip, the polo neck had served its purpose and been ditched - but a degree of attention to detail that most of us would find exhausting. For a brand, a change of hair style is a momentous decision, comparable to the launch of the chunky Kit-Kat, and you never know how it will play.

If you get it wrong, you have to endure days of bitchy comments from the press - Mrs Beckham has been unkindly compared with My Little Pony - but it doesn't matter. What a brand needs to do is provide a constant stream of new pictures and copy, which is why Diana's star faded so quickly after her death in Paris. For a few weeks after the fatal car crash she was everywhere, and then the absence of fresh images had its inevitable effect, to the surprise only of onlookers who failed to grasp the nature of this contemporary phenomenon.

With the 10th anniversary of her death approaching next year, you don't have to be a genius to predict that the Diana brand is due for a revival, and the Royal Family is already thinking how to package her effectively. Sadly for the princess, the process she began long ago passed from her control. She's been replaced by a younger rival, a single-minded young woman whose entire career now consists of being Mrs Beckham.