This is very interesting. I was especially interested in Diane's report on foal scours, as Bunny developed diarrhea while at Tufts and I feel they might have over-reacted. She had diarrhea a few times despite my worming Mira right after foaling, which I had heard would help prevent it. But I think you are correct that the foal's system is changing, and I also found it seemed to be sensitive to what the mare was eating (DOH!). The first time she got diarrhea was right after Mira was on grass for several hours. Shortened the grass time and fixed the problem - until the next time! Never seemed to bother her except for getting her rear end messy.
Little Ribble - Maybe if you edited the title of this thread to say something about "milk pH after foaling" you might get more responses. Maybe someone will even be able to test the pH for you.