I posted on this issue on another thread. I find it interesting. My thoughts (if you can possibly wade through it all 
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I think that in a the majority of cases (not all, depends on the family dynamic) that once the son grows up and especially when he has a partner and family of his own, there is nowhere for the mother-son, the caring parent-child relationship to 'go'.
Think about it - mother-daughter: daughter grows up, has family - if she and her mother are close (or even if they're not) a different but very familiar and 'stereotypical' (not quite right but can't think of better word) relationship tends to be built as a matter of course - two adult women, many shared interests in forms of family, children/grandchildren - they may share friendship or hobby interests, they gossip, they may shop together or enjoy the same tv programmes (!maybe steroetypical IS the right word after all!) - in short, the mother moves into a different but potentially still close relationship which still involves long chats, one-on-one time, being involved in the minutiae of life, etc. In short still having an 'entwined' life. This doesn't need physical closeness - I know many women who live far away from family but will spend regular time chatting in depth to their mums on the phone.
The typical mother-son relationship probably couldn't be more different in the way it changes. There are many exceptions to this, but in general - how many twenty/thirtysomething men share general interests (shopping, tv, food, chatting about family/kids, general gossip?) with a fifty/sixtysomething woman? How many men are eager (both before marriage/kids are on the scene and after) to pick up the phone and spend time chatting about their day, what the kids said, what happened with that rude guy at work? In the vast majority of mother-son relationships, it seems that part of the issue (and remember my stereotypes disclaimer guys) comes out of the typical differences in the ways men and women tend to communicate and the tendency for the mother to lose that everyday intimacy with a son rather than a daughter.
The mother knows that a fulll, intense everyday life is happening in the son's household, and her most effective way of being party to it (and her son's everyday life, like when he was her child rather than a man) is to communicate with the WOMAN there - her DIL. But perhaps quite naturally she resents that, and it's not what she's looking for. She wants everyday intimacy with HER child, and not one mediated and translated through someone else.
I understand that and can see how many women, when they are told to 'let go', 'cut the apron strings' probably think 'But it's not that!! I don't want to MOTHER him - I just want...' - and they cant really even articulate it, it's just that sense of being made part of the everyday stuff by their son. With this emotion, there's little even the most welcoming DIL can do to improve the sitution.
It would be my guess that the MILs who are genuinely enthusiastic, welcoming, and CRUCIALLY - not interfering in the early days of their son's relationship have a good chance of this sense of loss passing them by, because they end up getting the minutiae, the closeness, through the DIL from the start, and pretty soon it becomes plenty enough to plug the gap. They come to love the DIL quite naturally and see her as family, and without realising it, by osmosis, they get what they need in terms of the relationship with their adult son.
The ones who have problems are the ones who are determined to retain the feeling of what they once had through the son and he won't play ball. He's often oblivious (how many women come on here complaining that they have to remember his mum's birthday, they have to remind him to phone her or she'll be in a sulk not with him but with her, the DIL?) These MILs aren't getting what they need from the relationship but instead of realising that that's simply a function of their adult male son's personality, they interpret it as him withdrawing deliberately, being 'turned against them' or 'taken away' - they blame the DIL. They have to, because the distance being their SON'S default setting is too painful to accept.
Thousands of exceptions to this but I think it's a very common scenario. One poster has spoken of the 'girl preference' thing often talked of on MN - I think we would be naive to not recognise that this is part of where it stems from. I think many mothers look into the future and their relationship with their adult children and can see themselves chatting on the phone to a daughter for hours (because they do that with other women) but not their son (because they don't tend to do it with men). Or enjoying a day shopping or just hanging out (ditto).