I was reading Everett's paper http://rmp.aps.org/abstract/RMP/v29/i3/p454_1 on the "Relative State" Formulation of Quantum Mechanics, and i realized that this is not an interpretation experimentally equivalent to other "interpretations" like the Copenhagen.. These are different theories that give different predictions! For example, Everett's (elegant and simple) formulation of quantum mechanics is based on the hypothesis that everything is quantum mechanical, hence the observer and apparatus are all quantum mechanical systems decribed by the deterministic Schrodinger's equation. The Copenhagen formulation of quantum theory denies that, which means that it's something testable to experiments. If someone is able to design an experiment which proves the quantum mechanical nature of an apparatus and an observer, then he disproves the Copenhagen theory, hence Everett's theory will be the correct one.. Someone may object that proving the quantum mechanical nature of the observer is mission impossible. Indeed. But is this relevant? The different predictions are there. The fact that one cannot design an experiment, for practical reasons and not of principle, to investigate these predictions is another thing. I never saw anyone calling string theory a "different interpretation" of nature (..not implying ofcourse that the degree of "impossibility" is the same in the two cases! :) ). To my understanding, we have to do with different theories and should stop calling them different "interpretations".