Unfortunately its a narrow margin.
It doesn't matter. The ruling came down. "We the people" won. This should put us a step closer back to the "right" way of things--the government afraid of it's people, instead of the other way around.
The argument that was being made by pro-gun-elimination advocates was that the Constitution supposedly meant that a militia had the right to bear arms, but "the people" didn't. The problem there is that militias are usually controlled by the local governments (or they're declared terrorists and infiltrated and destroyed), which are controlled by the etc etc. Being that this section was put in there specifically because we'd just shed the British from our soil--and needed a lot of guns to do it--their "people shouldn't have guns" viewpoint would make the 2nd Amendment less than useless.
The Supreme Court very rightly decided that the only sane interpretation of the 2nd Amendment was that it was there to make sure that individuals could bear arms in order to be able to fight back against their government, should circumstances make this necessary.
This is especially important stuff since we've got King George in the White House. I'm still not wholly convinced he's actually letting go in November. I certainly don't want to see anyone get shot, but a few hundred thousand people showing up in DC asking politely with guns for him to leave office would probably not be such a bad thing.
Gosh, guess which area has a strict "you can't have a gun" policy.
Supreme Court upholds 2nd Amendment [PDF]