Like Charles Darwin's theory of the diversity of life,
Evolution by Natural Selection, Albert Einstein's theory of gravity,
General Relativity, has been around for a hundred year now, and it is still being tested by current independent studies based on real observations... without failing to produce accurate results.
Chandra X-Ray Observatory recently (as of April 14th, 2010) reported on how Einstein's theory stood up better to independent observations of gravitational effects in distant galaxies than its two new challenging theories; the
f(R) theory and the DGP gravity, do (calculations based on General Relativity correspond better to what were observed than calculations based on the other two theories do).
This doesn't mean that Einstein's theory has been positively proven to be 100% accurate, of course. Science is not in the business of offering 100% certainty. It is in the business of minimizing uncertainties. And the more theories like General Relativity and Evolution by Natural Selection withstand their (almost daily) tests against real observations and experimental data, the more confidence they earn in that they are the current best description of the natural phenomena they describe. And since the people that came up with f(R) and DGP gravity are good and real scientists, they aren't pushing for their idea to be given equal time in science class with the theory that beat them.... Unlike what those people who are trying to push for a religion in disguise as a 'science' to be taught in science classes in America regardless of how thoroughly it had been debunked like to do...