[0] http://www.bbc.com/news/science-environment-15523520 [1] http://www.bbc.com/news/world-europe-21693140 [2] http://news.bbc.co.uk/2/hi/europe/6338535.stm
https://en.wikipedia.org/wiki/Sunstone_(medieval)
IMHO, I think is sufficient evidence to say that it is reasonably likely they were using these.
Churches and monasteries had them in their inventories, but how does that prove they were used for navigation? It would be like churches keeping sextants under the altar. It doesn't make sense.
The shipwreck that contained the calcite was found far away in the Channel and dated 500 years after the vikings. And there's no record of it in any navigation books of the time.
It's myths and internet memes, being propagated over and over again. I don't doubt it'll be on HN again 3 months hence.
While I don't think the case has been proved its certainly one of those things where there's a reasonable chance of it being true.