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!"
"So what?"

Another example:
"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?