I have taken photos of the board and I've seen lots of classmates do it as well. I had one prof who absolutely forbid any photos in class, even if they were just of the notes on the board. I saw one student defy him and take photos anyway. After class he sprinted over to that student and demanded to see his ID, which he photographed and then left.

The poor student followed him all the way out of the room and down the hall, begging not to be reported to the associate dean's office!