Joel test: Do new candidates write code during their interview?

At first glance, I think it makes sense to ask candidates to write code. Not on paper, of course, but by using a real compiler/scripting engine. On the other hand, it may not necessarily be that much of a good test. Very often problems that may initially look insurmountable -- real-life problems -- just need to be put at rest for a while, until new inspiration comes along. At that point, the problem may even become trivial to solve. Joel's test could of course revolve around simplistic fizzbuzz questions. This type of questions will, however, not say that much about a candidate's real abilities. How to set up a code writing test that will accurately measure programming skills?