This is probably being given more attention than it deserves since it isn't quite as strange as people are making it sound: "Why was a 9th century Viking woman buried with a ring that says ‘for Allah’ on it?" (Washington Post); "Ring brings ancient Viking, Islamic civilizations closer together," (Science News), and similar reports.
They are all reporting on a scholarly piece from The Journal of Scanning Microscopies, of an article called "Analysis and interpretation of a unique Arabic finger ring from the Viking Age town of Birka, Sweden," for those who prefer the original paper to the journalistic summaries. The illustrations below are from that article.
Unpointed Kufic is always a challenge, and inscribed on glass even more so. Here's their rendering:
Interesting, but not that revolutionary. Rings with Arabic inscriptions have been found in several sites in Eastern Europe, and Birka, Sweden, was a key Viking trading center, so the ring presumably arrived as trade goods.
|Offa's "Dinar" (British Museum)|
And of course there was Ibn Fadlan, the medieval Arab traveler who visited the Volga Vikings in what is now Russia. He didn't like them much. Michael Crichton's novel Eaters of the Dead was a fictionalization of Ibn Fadlan's travels. A bad movie was made from it, The 13th Warrior.
So the Birka find is interesting, but not unprecedented. The trade routes across Europe are quite ancient; Ancient Near Eastern jewelery has been found in Scandinavia, and the Ancient Greeks knew Baltic amber.