It is possible to cause a sudden high-pressure release of carbon dioxide by inserting multiple Mentos into a container of Diet Coke. Mythbusters found this is caused becuse of nucleation as well as certain catalysts in the Diet Coke and Mentos themselves. The phenomenon is described in some detail by the General Chemistry Online! FAQ, which also attributes it to nucleation.
This does not appear to work with the Australian formula of the product. While there is a release of carbon dioxide similar to that observed on Mythbusters, the effects are nowhere near as spectacular.
Presented by Tom Millett
http://antoine.frostburg.edu/chem/senese/101/consumer/faq/mentos.shtml