In the meantime, Der Spiegel has a fascinating article on the Pirahãs of the Brazilian Amazon:
...But the language of the forest dwellers, which Everett describes as "tremendously difficult to learn," so fascinated the researcher and his wife that they soon returned. Since 1977, the British ethnologist at the University of Manchester spent a total of seven years living with the Pirahãs -- and he's committed his career to researching their puzzling language. Indeed, he was long so uncertain about what he was actually hearing while living among the Pirahãs that he waited nearly three decades before publishing his findings. "I simply didn't trust myself..."
His findings have brought new life to a controversial theory by linguist Benjamin Whorf, who died in 1914. Under Whorf's theory, people are only capable of constructing thoughts for which they possess actual words. In other words: Because they have no words for numbers, they can't even begin to understand the concept of numbers and arithmetic.
The full article is available here.