In an equally unsurprising decision, the Supreme Court decided that file sharing services may be sued if "they intend for their customers to use software primarily to swap songs and movies illegally."[1] This seems to me a necessarily messy but also logical conclusion to reach. It seems to me that the Sony Betamax argument would protect the creation of mp3s, but not the distribution of them. While I personally view file transfer as being morally equivalent to recording something off the radio, that is not the argument being made. Rather, it is being argued that the mp3 distribution service is legal because it can be used legally, despite the fact that it is marketed for illegal use. The implication is clear here. If you create a file sharing service and market it for all those legal uses and it just happens that most of the shared content is illegal, you would stand a much better chance in court.
One part of the decision does bother me though. The Supreme Court apparently took into consideration the failure to take "easily available steps to reduce infringing uses."[1] There are no "easily available" steps to reduce infringing uses that will have a noticeable effect on the availability or ease with which infringing content can be shared. Thus I worry that this inclusion will be used to widen the doors to what can be shut down far further than is rational or supportable. The fear that the EFF and others came to this case with could be substantially realized if this requirement is read too broadly, or too cluelessly.
[1] http://www.washingtonpost.com/wp-dyn/content/article/2005/06/27/AR2005062700471_pf.html