Update: My sister called Mars today ( 07/09/2008 ) and Mars told her that they reverted back to vegetarian alternative to rennet in UK (as the commenter indicated) and they never switched in US. So we can all enjoy our favorite chocolates. This proves the point that consumers can affect corporate decisions if they get together. I hope we can convince Frito-Lays to go back to vegetarian friendly rennet alternative.
According to this BBC new:
Mars starts using animal products (http://news.bbc.co.uk/2/hi/business/6653175.stm) Mars started using animal based rennet instead of vegetarian alternative.
Mars/Masterfood products having best before date after 1 October 2007 would contain rennet (animal product).
Here are some of the products made by them:
Bounty
Celebrations
Dove Chocolate
Flyte
Galaxy
Kudos
Lockets
M-Azing
Maltesers
M&M’s
Mars Bar
Mars Delight
Milky Way
Minstrels
Mars Planet
Skittles
Snixkers
Starbur st
3 Musketeers
Topic
Twix
Snicker’s Marathon Energy BAr
Aquadrops
Halal Consumerism 
Your information is outdated. After outcry in the UK Mars backtracked and reverted to the vegetarian rennet for its products that contain whey. (But Starbursts & Skittles in the US still contain gelatin.)
Thank you cybele for pointing this out.
my question ‘All type gelatin are haram?’
here is the link for chocolates that still contains animal products.
http://www.guardian.co.uk/uk/2007/may/21/lifeandhealth.business
EXTRACT
The company behind some of Britain’s top-selling chocolate bars has admitted many of its well known brands will remain unsuitable for vegetarians, despite reversing a decision to use animal extracts in its products.
While Mars UK has backflipped over the use of animal extracts in some of its chocolate confectionery – including Mars and Snickers bars – ingredients used in its other products, such as Twix, Bounty, Celebrations, Topic and Milky Way, will not be changed, meaning they will continue to be unsuitable for vegetarians.
Moazzam,
Thank you for sharing the guardian article, I will do more research based on it.
Regarding your question about gelatin, Gelatin by definition is an animal product (there are vegetarian substitutes like Agar-Agar and Tapioca that are used for similar purpose as a gelling agent).
Any gelatin coming from non-zabiha (not slaughtered in proper islamic way) animal or from haraam animals like (pig) is not appropriate for muslim consumption. (as per my discussion with Islamic Scholars)
Any product that is made in a muslim country (Malaysia, Indonesia, Pakistan etc) and contains gelatin is OK as per my understanding as animals in muslim countries are properly slaughtered.
Allah knows best!