It depends on the context, as 'so what?' is used in many ways, and varied according to the tone in which it's said.
As a stand-alone complete sentence, it's usually used in a surly or rude way, as in the following example:
"Hey, you idiot, where did you learn to drive? You nearly ran over my cat!"
"You're late, and dinner's ruined again!'
"So what? I'm not hungry..."
In these contexts, 'so what?' means 'so what's your problem?' or 'why should I care'?' Maybe this would translate as "E allora?" o "Embé?" What would an ill-mannered Italian say in such a context?