Thank you for your comments. A few things need to be said.
I realize I am not paying her to mind him but I also would have hired a CM if she had not been absolutely insistent on being the one to care for him. This balance of power is something that I am struggling with because while I am appreciative, it shouldn't make her entitled to being quite so intrusive in my daily life outside of that arrangement IMO. I may very well quit work if I find that once I go back she is still hounding me on the days that I am home with DS. I didn't agree to 2 days plus a free licence to phone and visit every other day of the week. I definitely want DS to see her but I personally think 2 days a week is really enough. My own mother is dying of envy because she lives out of town. IMO, my MIL has it pretty good.
Yesterday, she called to say she could take him in the afternoon even though we had a date set up for tomorrow and she had him the day before. I explained that while I was on mat leave, I was very grateful to her to be able to take him from me 2 afternoons per week, however, other than that I didn't routinely need additional help. I told her that above and beyond that she could call and visit with us if we were free that day. So while I think I may sound sour on MN postings, I think I am very generous sharing my time with DS, and very polite to MIL.
Finally, I think for her own sake, she needs other sources of satisfaction and reward in her life beyond my DS. In the near future, DH and I plan to relocate for work, and she has said she will be severely depressed when that happens (which I understand will be very hard). She has to cherish her role as grandmum but avoid the trap of reliving her motherhood vicariously. She spends more time and money shopping for him than I do. While many of you think this may be great, I tend to think it's slightly warped.
I definitely agree with the fact that MIL/DIL relationships are usually strained and this one is bringing out the worst in me. But there you have it. I will get mine one day when DS makes ME the MIL!!!