This is such a tricky one, we can choose our friends but we can't choose our family. But we do have to associate with them for the rest of our marriage, and if you don't have a good relationship then that's when it gets tricky.
I agree with citygirl, about confronting her, but try a bit of reverse phsycology. Instead of going for the throat, I always think buttering her up with a few compliments first helps like "you always look so nice," or"where do you get your hair done it suits you so much" you no that sort of rubbish, then when you have got her eating out the palm of your hand, bring the big guns out. ie "I notice you were critical recently when I did so and so, is there a reason for that?" Etc etc.
I would certainly try and sort this problem out before going for the throat, as you are lumbered with her for the rest of your life. You might just find out, she was sensitive to one little remark you made, and thats why she's got the hump. One apology might just do the trick, and then everyones happy.
My MIL has on the whole been pretty good, considering she's as nutty as a fruitcake,but she has always spoken highly of me, however, that doesn't mean to say she hasn't upset me occassionally or irritated me. Once when we were living abroad, she kept ringing us saying "when are you coming home, we miss you and are lonely". Eventually I said, "hey how about you ask how we are doing, and give your son credit for what he is doing, ie making a career for himself and family, instead of just thinking about yourselves". She didn't speak for a few weeks, but eventually she started to ask how we were doing, and to say how proud she was of her son!She still says the odd thing to annoy me, like how she prefers one of my sons partners to the other sons partner, which I don't think she should do, but on the whole she has be a decent old girl!!! Lots of luck, hope this helps a bit.
Snowy