I visited Piper at Vero Beach in April. They were very friendly and showed me over a couple of new aircraft on the ramp. However, the factory tours are only conducted on one day a week (Thursdays I believe) so there was no way to look over the manufacturing facilities as I was there on a Friday. I'm sure the other manufacturers will have a similar setup.
So, just contact Piper/Cessna/Cirrus/Mooney website and book up a tour well in advance or you will be disappointed if you just turn up at the gates !