I know that the Justice Department has the discretion to decide which laws they want to prioritize the enforcement of, but that's a far cry from a President just deciding by himself that a law is unconstitutional and choosing not to enforce it.

TPM:

Chief Justice John Roberts took a swipe at President Obama during oral arguments Wednesday, arguing that the president should stop executing the parts of the Defense of Marriage Act he deems unconstitutional rather than relying on the courts to pave the way.

"If he has made a determination that executing the law by enforcing the terms is unconstitutional, I don't see why he doesn't have the courage of his convictions," Roberts said of Obama, "and execute not only the statute, but do it consistent with his view of the Constitution, rather than saying, oh, we'll wait till the Supreme Court tells us we have no choice."

And here's Scalia:

Which is the equivalent of the government saying, yeah, [a law is] unconstitutional but I'm going to enforce it anyway... I'm wondering if we're living in this new world where the Attorney General can simply decide, yeah, it's unconstitutional, but it's not so unconstitutional that I'm not willing to enforce it. If we're in this new world, I don't want these cases like this to come before this Court all the time.

Technically the President has to follow the law. It's literally the Supreme Court's job to decide what's constitutional.

It's true that another approach to this would be to have Congress repeal DOMA, but that's not what the Justices are saying here, and of course we all know Congress wouldn't have done that anyway, and i like it better this way since the Supreme Court can now set the precedent that discriminating based on sexual orientation is unconstitutional.

shouldn't an understanding of what the job entails be a requirement for getting the job? how can the system be so broken that you can get appointed to the Supreme Court and have no idea what your role in government is? gah!

They're only asked their position on Roe v Wade.