MTV have posted a news item debating the possible meaning of the confirmed Indy 4 title via a Harvard lecturer. Here are some extracts from the article,

“Although crystal skulls were once displayed in museums across the globe (including the Smithsonian) as proof of ancient Mayan grinding technology, they’re actually all fakes, most likely manufactured in late 19th-century Germany. Zender said this makes the artifacts “as perfectly legitimate a subject for Indy’s fictional adventures as either the Ark of the Covenant or the Holy Grail.

[...]

hey’re not attached to any real ancient tradition. Every true believer has a slightly different take on them,” Zender sighed. “As with most romances, by the end of it, the skull will have to be destroyed and the [ancient] civilization will pass away. But we’ll just have to wait and see how.”

AICN in the mean time have pointed out a fascinating documentary on YouTube concerning a Mayan Crystal Skull discovered in the 1920s.