Hitchen's letter addresses the problem of American Christian creationists attempts to influence school curriculum. In my opinion religious and philosophical interpretations of scientific theories should be discussed in other classes, not while studying biology. Sarah Palin is a worrying example of the ignorance of USA people using religion for political gain.
The letter seems to adopt positions close to August Comte, who defined what has been positively given to mankind and rejected metaphysics. However, other philosophers have long ago pointed out that Comte's own position is metaphysical. Ludwig Wittgenstein extended this to separate blabbering about things unspeakable from accurate verifiable language. But what made him an authority of reality and limit so drastically what humans are able to think and speak about. Logical positivism is important but also a trap, a prison of mind, that not make Wittgenstein a happy philosopher a la Democritos.
Evolution is the big thing in modern worldview, but it is human concept largely based on the brilliant and difficult to understand work of Hegel (this is a big subject). When human egg becomes pregnant, we do not call the deterministic (teleological) growth and development of the embryo "evolution". But when planet Earth becomes impregnated by life, perhaps by the interstellar sperm in Sir Fred Hoyle's theory of the origins of life, and reaches the peak of biodiversity in Mesozoic era, we do call it "evolution" and try to understand it by observing both living and paleontological evidence like Charles Darwin did in Patagonia and Galapagos Islands.
Interestingly, the name of his brilliant and revolutionary scientific publication "The Origins of Species" remains - after 150 years of intensive research - an elusive goal. We really do not yet know why chimpanzee brains kept that block which was removed from our ancestors head leading to enormous and fast evolution, growth, development, of human brains.
As for religion and science, a value pair that Mr Hitchens seems to abhor - Big Bang theory was originally developed and discussed with Albert Einstein and others by a Catholic priest, Monsignor Georges Henri Joseph Édouard Lemaître, who did not improperly mix science and religion when applying general theory of relativity to cosmology.