It has long been accepted that a Norse colony was briefly established in North America at a place called L’Anse aux Meadows around 1,000 years ago. Two written accounts of that colony, together called the Vinland Sagas, date from several hundred years later. Differences between the two sagas, added to the fact that both were written long after any eyewitnesses had died, suggest that they should only be used very cautiously as sources of information about the settlement.
For example, neither of the sagas even hints at intermarriage between Norse men and American Indian women. However, a recently published analysis of mitochondrial DNA from Iceland strongly suggests that it happened at least once. More than 80 living Icelanders have been found with a genetic variation that probably came from North America before the time of Columbus. Because mitochondrial DNA is only inherited through the maternal line, this means that at least one Viking explorer brought an Indian woman back to Iceland with him.
I haven’t had a chance to read the full article yet (it can be downloaded here), but from the abstract, this looks like a very intriguing finding.