Actually, a lot of people who don't eat meat will eat sweets, puddings and other products containing gelatine - and they still class themselves as veggie because they're not eating meat per se.
A veggie I used to know offered me some Skittles sweets once, and when I pointed out that they weren't suitable for vegetarians because of the gelatine, she said "Well, I can't exactly see a lump of steak sitting in the pack ..." - it didn't bother her at all.
I also wonder how well Mars will publicise this, and how many people who don't eat gelatine won't realise that the ingredients have changed - people generally carry on eating the same things they always did without bothering to read the label.