It is easy to argue this either way. On balance, I would say that King's dream has largely come true, but not completely.
To argue that his dream has come true, we need look no farther than the fact that we have a President today who is partly black. In King's time, the idea that a person with evident black blood could ever become president would have been laughable. This, presumably, shows that we are more willing to judge people by the content of their character rather than by the color of their skin.
However, we are certainly not a nation where people of all races "sit down at the table of brotherhood" in any meaningful way. We are still badly split along racial lines with, for example, the Republican Party being almost all white and almost all African Americans voting Democrat (Herman Cain notwithstanding). We still see things very starkly in black and white terms, which is why a half-white president is seen as a black president.
We have fulfilled much of King's dream. But we still lack the kind of true understanding of one another (rather than simple legal tolerance) that King hoped for.