Yes I know its lovely to see longhorned cattle around, but the photos that alerted me are on Megalithic Portal, and are pretty terrible, the stones are only 3 to 4 feet high and the cattle are immense, a holed stone has been smashed, whether it was the cattle I don't know.
http://www.megalithic.co.uk/article.php?sid=560

The main thrust of the argument is of course the fencing of the moors which is seen as very unfriendly, you would know more about that ;), whether as TJJ says that fencing round the circles would help I don't know - lots of grumbling from photographers and people who want to get into the circle....
If the following is true, permanent enclosure of the moors is writ large, last time that occurred was probably the 18th century...

"New Higher Level Stewardship agreements have been signed that cover parts of Watch Croft, Carn Galva, Men-an-Tol and Lanyon Quoit Crofts. Extra fencing will be installed and grazing will mostly begin after next May."

It's also been covered here of course.....

http://heritageaction.wordpress.com/2011/03/24/cattle-damage-at-tregeseal-stone-circle/