Seems like almost everything to come before the court comes back with a 5-4 ruling, at least everything important. Ideology is wired into the justices as much as it is everyone else.
I read somewhere that the prevalence of narrow victories is by design (or evolution maybe): 1. the court chooses what cases to hear and is unlikely to spend time on anything they all agree on; 2. the body of precedent is large enough so that lower courts aren't making decisions the court would overturn with a wide margin; 3. the parameters of a majority opinion are written to get 5 of 4, anything else is overkill or watered down.
This one wasn't a traditional liberal/conservative split, though, so if it's ideological, it's on a different axis. The majority opinion was Scalia/Thomas/Ginsburg/Breyer/Souter, i.e. 2 conservatives and 3 liberals. The dissent was Stevens/Rehnquist/O'Connor/Kennedy, i.e a liberal, a conservative, and two moderate conservatives.
