I just got this info from Wikipedia:
http://en.wikipedia.org/wiki/Mars_Bar Animal products controversy
In May
2007 Masterfoods announced that Mars Bars, along with many of their other products such as
Snickers,
Maltesers,
Minstrels and
Twix would no longer be suitable for
vegetarians because of the introduction of
rennet, a chemical sourced from calves’ stomachs used in the production of
whey.
[6] Masterfoods confirmed that many other products such as
Easter eggs and ice cream would also be affected.
The decision was condemned by several groups, with the
Vegetarian Society stating that "at a time when more and more consumers are concerned about the provenance of their food, Masterfoods’ decision to use non-vegetarian whey is a backward step".
However, Masterfoods later abandoned these plans, stating that it became "very clear, very quickly" that it had made a mistake.
[7]