I just came back from Italy two weeks ago (read all about my exploits here!), where I ate my share of pizza. Indeed, the best way to eat in Italy is to grab a bite of pizza sometime mid-day and let it carry you to a full dinner later on. (Or you can have the big meal at noontime and get a slice later, if you want, but I found this less than conducive to sightseeing.)
The aforementioned pizzerias are usually sit-down establishments. More common are pizza al taglio, or pizza by the slice shops. Just about every city block in Italy has at least one. Here, they make big, square pizzas that sit behind glass, and you point to one and tell them how much you want. It is sold by the etto, or 100g. One etto is a good snack, and two would fill me up nicely. They cut off about that much, weigh it, heat it up in the oven (if you want them to), and then serve it to you folded in half like a sandwich and wrapped in paper. It was usually about 2000 lire–a little less than a dollar–for an etto, making it a great deal.
My understanding is that some places in Italy (Naples, maybe? Didn’t make it there) restrict what can be called “pizza margherita”, much like the German beer purity laws. Usually, this includes having to use “real” mozzerella cheese from buffalo milk. Most of the margherita I had was really good.
I saw the same sort of sausage pizza that Phoebestar saw, maybe in the same shop in Florence. (Over the river near Piazza San Spirito?) The sausage was in little greasy balls about an inch wide, meticulously arranged on the points of about a two-inch grid. The sausage was rather pink; I think the Italians are simply more willing than we are to take the small risk imposed by undercooked pork. (I ate it, but I admit I was a little antsy. I was more worried about the extra grease I was adding to my arteries. 
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The most bizzare pizza I saw was either the aforementioned pizza frutta di mare (best seen around the Cinque Terre) or the one at Pizza Rustica Europa, not far from Termini in Rome, that was simply a pizza crust smothered in Nutella. Mmmmm.
It used to be that a lot of restaurants in Italy emphatically refused to serve anything less than a full meal of at least antipasti, primo, segundo, and dolce. That isn’t really the case anymore, though; I would often order just an antipasti and primi and a half-liter of wine, and I never got a funny look or comment. The exception was in Florence and Siena, where the Tuscan meats were just too good to pass up.
Dr. J