Most NYC pizza places that I've observed are using par-cooked sausage. I've never seen a container of raw sausage in the prep area.
Most NYC pizza is cooked in stainless steel deck ovens running at 500-550 degrees. These pizzas often take as long as 12-15 minutes to cook. Possibly enough time to cook raw sausage, but again this has not been my observation.
Wood-burning and coal-fired ovens running at upwards to 700 degrees will cook a pizza in 3-4 minutes. I'm not sure if that's long enough to properly cook pork sausage but would guess not. I'll take a look at what they're doing on my next visit. I'm betting it's par-cooked.
Neapolitan style pizzas cooked in wood burning brick ovens cook in under 90 seconds. I have doubts that this is long enough to cook sausage properly, given that it's cold and sitting on a cold skin with cold cheese and sauce. These pizzas cook only long enough to have the fresh mozz just melted. I would suspect mozz melts more quickly than sausage cooks.
Chicago deep dish places use raw sausage, but their pizza cooks for 20 or more minutes, and it gets very hot in that pan of sauce and cheese during that time. I'm guessing they use a sausage without a high fat content, otherwise they've have a pastry shell filled with pork fat (not a bad thing, mind you, but probably not very good in the context of pizza, deep dish or not).
I cook pizza at home in about 4 minutes. For this, I cook the sausage out of the casing pretty much until done. It has time to heat up on the pizza, and that's about it. There is plenty of flavor in the sausage. I don't subscribe to the "fat=flavor" thing. Plenty of food has flavor with no animal fat...dough, sauce and basil among them.
<message edited by tommyeats on Mon, 04/26/10 6:19 PM>