While I agree that Sardinia and Rome share similarities and face some of the same problems with the Mezzogiorno I do not consider these regions part of Southern Italy. They are their own distinct entities with their own historic memories and should be in control of their own destinies. For me, Southern Italy’s boundaries are those of the former Kingdom of the Two Sicilies, which includes the regions of Abruzzo, Basilicata, Calabria, Campania, Molise, Puglia and Sicily.
Having said that, I would like to stress that this also includes the territories partitioned and rezoned into the province of Lazio in 1920's. For example, Gaeta is now technically part of Lazio, but this area historically belongs to Campania's Terra di Lavoro and is an important part of Southern Italian history. In fact, Gaeta is where the Bourbons made their heroic last stand against the invading Piedmontese. Amatrice, Cassino, Cittaducale, Sora, and others belong to the South as well, and in my humble opinion any talk of an independent Southern Italian nation must include these lost territories.
As for who is Southern Italian, the most straightforward answer I can give to this question would be to say the indigenous people (Calabrese, Neapolitans, Sicilians, etc.) of the Southern Italian regions. This, of course, includes the millions of immigrants and expatriates from Southern Italy found around the globe. On the other hand, migrant workers and refugees coming into the homeland, even those who are law-abiding and speak the local vernaculars perfectly, are not Southern Italians. Even though I live in the United States I'm still Southern Italian because my ancestors hail from Napoli, Sturno and Catania in the same way, for example, the Chinese living in Palermo will always be Chinese. By law they may be citizens but they are not Southern Italian. Certainly, some kindred people (the Arbëreshë, for example) are assimilable, but I believe that a people create a culture, not the other way around, so it's undesirable to change the ethnic composition of the regions, especially if we wish to preserve our distinct culture and heritage.
~ Giovanni di Napoli, August 5th, The Feast of the Madonna della Neve
Addendum (added December 31, 2010):
Chinese schoolhouse in Southern Italy helps children remember origins