I have to say that Kristi and Jennifer have the right idea.
I have the mother who just doesn't know what to say to my husband. She LOVES him, she just doesn't think he wants to talk with her because he's got a different personality. When she calls, she is polite to him but really just wants to talk to me and/or her grandson.
I wouldn't really take your MIL's actions to heart. Give her some time to warm up, keep being nice and try to slowly engage her into conversations. What are some of her hobbies? Anything that you enjoy or would like to try? Compliment her flower arrangements, her garden, whatever. I found that this has been best for my husband and mother, they both like country music...so that is what they talk about; for now...it's something they have in common besides their love for me.
Whatever you do, don't push the relationship with her and don't threaten; as long as she is being polite in her not talking to you. Now, if she turns out to be an "evil twit" as I like to say, :) and you use threats and keep her from your son, it's your husband and son that will feel the hurt the most and it will just poison your relationships in the family.
As far as her saying that you spoil your son... maybe that is the best she can do in conversation right now. Not to mention that that is what people do...especially when they've forgotten what it was like when they were new mothers. Times are different now so just keep that in mind. In fact, I heard a great comeback to "those" people: Smile and say, "Thank you for your input. I'll(have to)think about that". Or, "I will keep what you've said in mind, but for now I'm still learning this motherhood thing and the pediatrician says he's doing well so I must be doing something right!" :) and kind of laugh it off.
Basically, be the bigger person, take the high road and all that jazz, she'll come around. And if she doesn't, it will be her loss and everyone around you will see that you've tried. Good luck!