I second doom's blog -- it's very good.
Obviously, there are a lot of questions to ask during a visiting weekend. IMHO, learning about the grad student's experience is at least as important than learning about a professor's research. I think an important (but not often enough asked) question for professors you're interested in, is where their students end up. At one place, it sounded like most students ended up in finance. In another, they ended up in industry, government labs, and one even became a professor. Interpret and use that information as you will.