For an explaination of what is going in with the coke and the Mentos, here are three websites that may help

http://www.eepybird.com/science.html

http://www.sciencebuddies.com/science-fair-projects/project_ideas/MatlSci_p023.shtml?from=Home

http://antoine.frostburg.edu/chem/senese/101/consumer/faq/mentos.shtml

These chemists are saying that the primary cause is physical, not chemical.  Their explanation: nucleation sites. If you have a liquid that is supersaturated with gas (like soda, which is pumped full of carbon dioxide), a nucleation site is a place where the gas is able to form bubbles. Nucleation sites can be scratches on a surface or specks of dust – anywhere that you have a high surface area in a very small volume. That’s where bubbles can form.

Mentos seem to be loaded with nucleation sites. In other words, there are so many microscopic nooks and crannies on the surface of a Mento that an incredible number of bubbles will form when you drop it in a bottle of soda.  Since the Mentos are also heavy enough to sink, they react with the soda all the way to the bottom. The escaping bubbles quickly turn into a raging foam, and the pressure builds dramatically. Before you know it, you’ve got a big geyser happening!

From Eepybird website

Eepybird also has a link to a pdf file which gives a different launch mechanism for the Mentos mints. Worth a look.