I'd completely buy her as Lady Sif - but I'm not sure that she can swing a sword too well.

But, honestly, I'd probably watch Natalie Portman eat a plate of fava beans. She's not unattractive, don't you know?

(Edit: Apparently she's been cast as Jane Foster - which I think's odd as I'd read that the majority of Thor was taking place in Asgard, with Earth only showing up right towards the end - guess I need to go hunting online for an updated script...)