People can be so superior about this that I was rather pleased to find recently that the Oxford English Dictionary offers historical support for those who write 'avocado's' and 'tomato's' on their price tags. The apostrophe originally marked the omission of an e and was equally common in the nominative plural (eg potato's for potatoes). Then it was gradually extended to all possessives and its use in plurals fell away. Even in 1876, though, you can find a grammarian fulminating against a form that would now be regarded as unassailably correct: 'It is an unmeaning process to put the apostrophe after the possessive plural s (as birds') because no vowel has been dropped there.'
I doubt if the matter is settled yet and, certainly, if you read tabloid papers closely, you can see evidence of new grammatical experiments with this troublesome mark. The tabloids have always been good at making a little Royal Family go a long way, conferring on associated commoners a sort of honorary royalness, but I noticed a while ago that this had started to be reflected in the grammar of headlines, as well as in the coverage itself.
'Di mum to wed again', read one tabloid banner when the Princess of Wales's stepmother fell for a French count. 'Charles pal in court', read another, covering the conviction of a man Prince Charles had probably brushed past at prep school.
What was interesting in both cases was that the disappearance of the apostrophe transformed a simple description into a title. 'Di's mum' is rather ordinary, after all - there are hundreds of people that can accurately be described as 'Di's mum'. 'Di mum' is altogether different - it sounds a bit more like Prime Minister or Chief Equerry.
I would not want to become too grand about this, because it probably has as much to do with the British terror of apostrophes or the size of the headline type, but both phrases convert proper names into adjectives. 'Di' does not refer to a person here, but a condition, which redefines anything it comes into contact with. Royalty provides its own model for these constructions - Queen Mother may offer a similar trick - but the tabloid version is more complex.
As far as I know, Sophie Rhys-Jones has not yet been referred to in print as 'Edward girl', though she might have preferred that to 'Blobby's girl' - the most popular epithet2 on the day the news broke and a reference to the fact that the spotty superstar is promoted by the public relations company she works for. The fact that Mr Blobby still requires an apostrophe suggests either he yet to reach the status of royalty or that tabloid journalists are more sensitive than we give them credit for being - 'Blobby girl' would hardly have been fair.
There is another possibility - one that suggests even subtler nuances to the tabloid apostrophe. In the unlikely event of the actor who inhabits Mr Blobby falling from grace in some way (drink- driving or minor shoplifting being the Equity-approved methods), I'll bet good money that he will be referred to as 'Blobby man'. Similarly, in the case of 'Di mum' and 'Charles pal', there is a faint coldness to the phrases, a feeling that the paper wants to make it clear that it does not blame Di or Charles for these regrettable connections.
The withheld apostrophe makes the association contingent3 and impersonal. Where 'Edward's girl' is cosy and approving 'Edward girl' merely remarks that a little reflected glory once shone on this person. The apostrophe is not so much a knot here, more a medal with its ribbon, conferred only on the respectable.
1 From the Greek apostrophein, to turn away: apo, from; strophe, a turning.
2 From the Greek epithetos, added: epi, on; tithenai, to place.
3 From the Latin contingere, to touch together.Reuse content