SCOTUSblog is liveblogging.
UPDATE: By a 5-4 vote, with no plurality or concurrences — i.e., the five justices in the majority all agreed on the same rationale — the "Second Amendment protects an individual right to possess a firearm." (That quote is from SCOTUSblog, not from the opinion.)
UPDATE 2: Here’s the opinion and the dissents (PDF). Scalia wrote the majority opinion, which was joined by the other four conservative justices. There were two separate dissents, one by Stevens and one by Breyer; each dissent was joined by the other three liberal justices. So, like I said, a clear 5-4 split, with no muddying of the waters by multiple plurality opinions and such.
If you don’t want to read the whole thing, some relevant quotes from the majority opinion can be found here. And here’s an initial analysis from SCOTUSblog’s Lyle Denniston — although, as the Volokh Conspiracy’s Orin Kerr says, "the details of the opinion are critical; it will take a bit of time to read the decision to get a sense of what it means."