Mars made a post on their website apologizing for the ingredient change that introduced young calf stomach lining into many of their chocolate bars. They are rolling the change back and most, but not all, of the products listed will be vegetarian again. Read here for the announcement, schedule, and which products will have what status: http://www.masterfoodsconsumercare.co.uk/veg_status.asp
All said and done, I think they handled this rather well. They certainly don’t have to cater to a vegetarian minority, but they were nice enough to do so.
Note: Twix, Bounty and Celebrations will continue not to be suitable for vegetarians.