The Wall Street Journal LawBlog posted an interview with Justice Kennedy.  Here's what Justice Kennedy said about how the court chooses cases:

Of the 9,000 [filed each year] we mark about 500 for discussion. From the 500 we discuss, we should take about 100, 120. Lately we’ve been taking only 80. There’s not a lot of emotional or intellectual capital expended arguing over whether we should take the case.  If it’s a really important case and we feel badly that it wasn’t taken, there will be another one [sooner or later] on the same issue.