My nephew has autism. Does that increase the chances that my own future child will have autism?

Maybe, but not much. Genetic influences with autism are actually quite strong--identical twins have a nearly 100% concurrence of the diagnosis in some studies. However, there is no single gene, and inheritance is quite complicated. Here's a good overview: http://en.Wikipedia.Org/wiki/heritability_of_autism.
Probably not. The cause of autism remains unclear. However, there is no current evidence for a genetic link.