My friend Jim Ho (a former clerk for Justice Thomas who's now an appellate lawyer at Gibson, Dunn & Crutcher) reports that Justice Kennedy is "16 for 16 in 5-4 cases so far" this Term.
Justice O'Connor, he further reports, "never had a perfect record in 5-4 cases (not countering her partial service during [the 2005 Term]." That, of course, stands to reason: She and Justice Kennedy were both swing voters then, so sometimes it was Justice Kennedy who made up the majority while she was in the minority. Now that Justice Kennedy is the one man at the middle of the Court, generally speaking as he goes so goes the majority.
Note also that now that Justice O'Connor and Chief Justice Rehnquist have left, one recurring 5-4 pattern in which Justice Kennedy was usually in the minority -- the sentencing/jury trial cases, in which Rehnquist, Kennedy, O'Connor, and Breyer routinely dissented -- may end up being a 6-3 pattern; see Cunningham v. California, the most recent such case, in which Kennedy, Breyer, and Alito dissented. So in part Justice Kennedy's perfect 5-4 record might stem from situations such as this, where his view is getting less popular on the Court and is thus being excluded from the 5-4 count. Still, one can't deny that in many cases he is the Justice to watch. SCOTUSblog's StatPack has more details. In 7 of the 16 cases, Kennedy joined the four generally more conservative Justices, in 5 he joined the four generally more liberal Justices, and in 4 the Court split in other ways.