Im guessing Sharon aka Athena is having the vision because she is the mother to Harrah. The six that is locked up on Galactica , the one that had a relationship with Baltar and helped start the attack on Caprica, is having the vision as well. The only other person that has seen it is Roslin.
I would say the vision is one big ass foreshadow to whats going to happen to Harrah. The opera house has been alluded to in a couple of flashback/hallucination/visions throughout the show. Like when Baltar crashed lands on Colbalt. He hallucinates he is in an opera house with Six. And he sees that baby crib. Also when Dianna saw the faces of the five she seemed to be what looked like an opera house. There maby more that I am forgetting.
Of coarse it all could be BS and the shows production staff was just reusing sets
Last nights show was great. I can't wait to see how Adama reacts when he finds out Col. Tigh is a Cylon. Also it should be interesting how the Cylons react when they find out that the Six leader chick was killed. And I would not be surprised if Gaida were to start singing All Along the Watchtower