Here is an excerpt from a message I sent to the list last week, which argues that nihilism is not an appropriate response to multiverse physics.
As far as the issue of human action and free will, here is how I look at it. There are really two issues. The first is that in some sense the multiverse makes our actions deterministic. That is, there is no longer any true unpredictability in what we do, because we do everything in one universe or another. So how can we have free will if there are no choices? Well, this problem has been considered many times in the philosophical literature going back hundreds of years (where it was asked how free will was compatible with God's omniscience). Recent works by Daniel Dennett, his books Elbow Room and his new book (which I haven't read) Freedom Evolves, discuss how free will can be said to coexist with determinism. The basic idea is that the acting out of deterministic processes and the considerations involved in making a free choice are two equally valid ways of explaining the same phenomenon, at different levels of description. These books could be good sources to explore these concepts further. The second part of the problem is specific to the multiverse model, which is, even assuming that in some sense you have free will, what is the practical point of acting, since your decisions will be in effect cancelled out by being done differently in other universes? Larry Niven's science fiction short story All the Myriad Ways explores the problems which sweep society when a technology is invented to visit parallel universes, leading to a widespread surrender to nihilism and social ennui. However this perspective ignores the concept of measure, where some universes are more prominent than others. Although you may make different choices in different universes, the probabilities are not equal. Your decision making processes influence the measure of the universes in which your different choices occur. By giving matters careful thought and making wise decisions, you can maximize the measure of the universes in which your choices have good outcomes. This justifies the necessity of careful choice and eliminates the descent into nihilistic horror and despair. Hal Finney