If she had dressed up, would the media then have just said she'd obviously taken Stephen Fry's comments at the BAFTAs to heart? I suspect after the attention on her clothes then that she'd have been damned if she did and damned if she didn't for the Oscars.
Somewhat derailing, but I spent four years going quite frequently to formal events with varying dress codes, and it always got my goat that the dress code was expressed in terms of what the men wore. What the heck does a 'lounge suit' mean in terms of a dress?
Also, some of the men bend the definition of 'black tie' pretty far...