Regarding string theory: how to refute the argument "if it cannot be tested experimentally then it is not science"? 
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What differs string theory from philosophy or religion? 

I find a lot of people disbelieve in string theory, saying that since it cannot be tested experimentally then it is not science (quoting Feynman who has said something along that line), and that it is a complete waste of time, and it should not be funded...etc. 
My question is how to convince those people that string theory is real science and real physics? and how to reply to the argument that if it cannot be tested experimentally then it is not science? where is the flaw in this argument?
Note added: Please if your answer is about attacking string theory or saying that it is science fiction and the like, then do not bother answering.
 A: To treat the question in the original title ("True or false: If it cannot be tested experimentally then it is not science?") seriously I'm going to say "Sorta".
Assertions that are forever beyond experiment are not science. It wouldn't matter if they are built on the mathematical foundation of science. If there is no way they could, even in principle, be put to the test then you have theology or philosophy.
String theories1 aren't in that category.
Instead they are very, very hard to put to the test. Essentially impossible given the current state of experimental technology and of the theories themselves.
Feynman's objection was that the one easily testable prediction that comes from them--large extra dimensions--is manifestly false and requires a somewhat artificial patch--make them small and compact--to keep the theories in agreement with the world we know. That's not showstopper, but it is a little awkward.

There is a rough historical parallel in the state of atomic theory in the late nineteenth century. There were some pretty good reasons to believe that stuff came in discrete packages, but no really solid proof.2
That situation was largely resolved when a obscure German-born physicist name of Einstein thought up a way to magnify the otherwise unreachable small effects predicted by the theory to a level that the instruments of the time could observe.
Which points the way for string theorists to calm the agitated waters a bit: put some more time into figuring out experimental signatures that are accessible at foreseeable energies. And feel free to take "foreseeable" to include fairly blue-sky projects like muon colliders. 
In the mean time the absolute, driving certainty that some well-known personalities exhibit to the mass media look a little odd for scientists.

1 And the multiplicity of them and the multiple free parameters in them matter: they make this a large family of theories with many different possible behaviors to try to sort out.
2 Come to think of it the luminiferous ether was another theory outside of experimental reach in the late nineteenth century that was put to the test in the early twentieth centruy. That one failed the test, which points out the importance of experiment in sorting these things out.
A: The best answer to this question is Lubos Motl's answer to this question: What experiment would disprove string theory? . It has 61 up votes and in summary says that string theory would be refuted if any of the following experimental or theoretical results were obtained:


*

*Lorentz violations

*Equivalence principle violations

*Information loss

*Mathematical inconsistencies

*Quantum mechanics violations

*Particles exceeding the speed of light

*Fundamental constants varying

*If mapping the vacua landscape did not find our universe

*If supersymmetry and extra dimensions are not found up to the Planck scale


The bottom line is that the theory already is effectively well established since it predicts general relativity, quantum mechanics and the standard model of particle physics plus whatever extensions to the standard model that are needed.
A: There is the assumption
1)  that "it cannot be tested experimentally
2)Implicitly, that mathematical models of nature are "not science"
As far as 1) goes the same might have been said when Maxwell proposed his equations binding the diverse observational formulae for electricity and magnetism into an elegant mathematically whole.
Later of course numerous tests came up.
String theory equivalently, in my opinion, binds the standard model, which is solidly based on experiment, with general relativity a feat that took decades of theoretical research after the Kaluza Klein effort which only dealt with electromagnetism and general relativity. Of course particular models based on string theory will be in the end rigorously tested either with data already existing or with future data.
As I see it, string theory is a whole branch of mathematical physics, and its relation to the final model that will emerge is analogous to the relation of differential equations to Maxwell's equations.
Does that mean that one should not study string theory and search for models based on it, and that this is not science? What nonsense. After all it was Neuton who created calculus for physics needs, a whole branch of mathematics. Was he not pursuing science?
so we come to 2)
Of course efforts to describe nature mathematically are science, as long as they are rigorous and are not refuted by the data. It is nonsense to think other wise. Of course there will be blind alleys and frustrating back trackings, but that is the way the understanding of physics progresses.
A: ''I am basically asking how to refute the line " if it cannot be tested experimentally then it is not science/physics" ''
This is the argument of those believing that Popper (cf. http://en.wikipedia.org/wiki/Karl_Popper) said the last word on the matter what science is. However, his views are not the only possible view on the matter; see the section on ''Criticism'' in the above Wikipedia link. In particular, there is another influential view about what science is, by Thomas Kuhn, which is far less narrow than Popper's.
As regards string theory, some answers to your question might be here: 
What differs string theory from philosophy or religion?
A: if someothing can not tested empirically then is MATHEMATIC :d :d providing equations are right
now, a little more serious, to say someone has invented a 'physical theory' this theory needs to be empirically testable, otherwise would be science fiction
