An interesting fact that I am assured of by a native Neapolitan source is that Neapolitan "dialect" and Portuguese are mutually comprehensible to the point of being identical languages. Albert Herring thinks not, adding that Neapolitan has historically been influenced by Catalan and Castilian (That's Spanish to you, fo', or Castilliano! as my friend Dan used to exclaim.), but ain't that the truth for all of Italy? (Evident in the use of the feminine third-person singular as the polite second-person singular. If this mystifies you, it means that you don't say "you" to your boss, you say "she", dig?).

What does this really mean?

As "dialect" in the context of the Italian peninsula means a language that is not Italian, nor particularly comprehensible to one who only speaks Italian, nor comprehensible to one who speaks one of these other "dialects", this means that there are a bunch of Italians who are basically speaking Portuguese.

The level of usage is, on the other hand, variable. As use of "dialect" is a class-marker of the lower classes, it tends to be eschewed by those who aspire to high-class. This means that it enters the speech of "proper" Italian speakers more as a developed set of idiomatic words and phrases than as a full language. Nevertheless, in the case of Neapolitan, it has a written form, easily distinguished from Italian; moreover, there are those who speak it as their first language, and are relatively ill-at-ease with Italian.

Surely you're joking, Mr. Pedant!

Well, for the specifics of Neapolitan, and its relationship to Portuguese, I have only a single source. My source prides herself on her relatively low level of Neapolitan, claiming not to understand the speech of the "lower classes", and those from the more rustic areas surrounding Naples (she is from the centre). So, in summary, I'm not joking, but I may be somewhat credulous.