While the general trend is for more products to become veggie friendly, even vegan, Mars have changed their recipes to include rennet, stomach juice from animals. From 1 May 2007 many of their most common pseudo-chocolates are no longer veggie.
To their credit Mars did contact The Vegetarian Society to warn them, so at least they are being fair and honest about this.
(Footrant: I stopped eating any confectionary that wasn’t organic or fair trade over ten years ago. Mass-market chocolate production involves slavery, pollution and violence. Vegetarians should avoid Big Chocolate products because they’re made by child slaves, not just because they’re made with rennet. Most Mars, Cadbury and Nestle ‘chocolate’ products are utter junk anyway.)