The Easter Island material is just the best known. All those archaeologists going there and saying "gee whizz".
How about just one?
The fact is, Rongorongo is some kind of writing system, just as quipu is a recordkeeping system (and maybe a writing system), but their secret is lost. At least in the case of the Rongorongo its purpose was known to the Easter Islanders themselves, who freely admit that there was never broad literacy in their pre-contact society.
from TES website — [snip] As an adjunct to Garn’s correspondence we take pleasure in presenting a reprint of one of Fell’s most famous Rongo-Rongo decipherments (originally appearing in ESOP, Vol. 19, 1990, pp 278-280). He deciphered the inscription shown in the background of a painting by Paul Gaugin of his Tahitian mistress and entitled “The Many Ancestors of Tehamana.” Fell suggests that the Easter Island script was long known in Tahiti, but concealed from the European administrators of the island. [/snip]
link to a page linking a nice blowup of the Gaugin painting:
http://popstuff-2010.blogspot.com/2010/10/new-gauguin-exhibition-at-tate-modern.html
wiki-wacky discussion:
http://en.wikipedia.org/wiki/Rongorongo
meanwhile, someone at Omniglot claims it was invented after the Spanish conquest in 1770:
http://www.omniglot.com/writing/rongorongo.htm