The Bruce/Natasha thing seems a little bit more like it's actually happening: Ruffalo and Johansson have both confirmed separately that each of their characters have a romantic plot with *someone* in this film. I don't know how far they take that plot, whether they actually get together, or there's just romantic interest shown, but all the spoilers I've read seem to imply at very least the latter.

I read a leak of the first ten minutes of the film that says explicitly that they are dancing around each other and Bruce is afraid to get involved with her because of what the Hulk did on the helicarrier

(Also, go look at that cap in the Tower again-- Natasha's hand is on Bruce's shoulder there, as well)