No one knows if Portman is being eyed to play Amora the Enchantress, lover and antagonist of Thor up in Asgard, or if she'll be playing a "real world" love interest of Donald Blake. The latter would suit the delicate Portman much better ... but hey, maybe playing a Nordic goddess could finally allow her to cut loose. It feels like it's been awhile since Portman wowed audiences, and maybe the Marvel Universe is just the place for her to do it.
Frankly, I would have preferred to see her as the Black Widow in Iron Man 2 over Scarlett "isn't my cleavage fantastic?" Johansson. If you're going to pick a Bolyen girl, you might as well pick the one who emoted the most, and the one who looks the most Eastern European. (To be kind, Johansson would be a better pick for the Nordic Asgard. Any way we can reverse the casting at this late hour?)
Meanwhile, we still don't know who will actually play Thor. Apparently, Marvel is leaning towards casting an unknown blonde -- Charlie Hunnam, Tom Hiddleston, Liam Hemsworth, and Joel Kinnaman are now in competition with the already reported Alexandar Skarsgard. At least they're no longer hosting open calls, and leaning towards men who have a bit of experience in British and Danish drama.