|MadSci Network: Earth Sciences|
A simple answer would be no. It is highly improbable that dinosaurs, or really, any extinct life-forms could exist on an "undiscovered" island. However, it is not utterly impossible. This is not to say that you should run and pack your bags, grab your binoculars and head for Isla Sorna. Though there are some phenomenal discoveries of living plants and animals that were once thought to be extinct. Some such examples include the recent discoveries of metasequoia (dawn redwood), and the bony fish known as the coelacanth (found off of the coast of Madagascar). Science is not good at "proving" something does NOT exist. Science is excellent at describing things that DO exist. Is there an undiscovered island? Well, we don't know (its undiscovered). Is it probable? Probably not. With the advent of remote sensing (infrared, satellite photography, etc.) geographers, and geologists have looked extensively at the earths surface, there is little area that has not been studied to one degree or another. It is not likely that there are undiscovered islands that are the big enough to support dinosaur life in the form of the extinct critters as we commonly think of them (though one can argue that there descendants are plentiful: they are all birds).
Try the links in the MadSci Library for more information on Earth Sciences.