Action Figure Insider has posted a promotional flyer from a Japanese company which makes trinkets and whatnot.
The image shows a variety of Indiana Jones-themed stuff (the Ark, the Hovitos skull, the Grail) and, amongst the loot, is a Crystal Skull from INDY 4.
BEWARE OF SPOLIERS IN THE IMAGE BELOW!!!
Yeah, so, I suppose this pretty much illustrates what we've been hearing for a while now...the Crystal Skulls are extraterrestrial in nature. At least, that sure looks like an alien skull to me...not that I've ever actually seen one in person.
Lucas has mentioned using some sort of MacGuffin in the film...so there could be a margin for error here. But, every detailed report I've heard confirms the skull's alien backstory, so...
You can read the original article (which has a few more details) HERE!
Thanks to Jaromir for the heads up about this.