Winston says, “We are the dead” to convey the reality of his and Julia’s situation. In going against the Party, even just through their initial relationship, they are condemning themselves to eventual death. Recall how the Party has a tight grip on everything and everyone in Oceania. They keep watch on people in their homes through telescreens; they monitor the words that people speak and write; and they even punish people for thoughtcrime—thinking unorthodox thoughts. So while Winston is aware that he and Julia are physically alive in this moment, he also knows that, with the Party’s power, it is just a matter of time until they catch on to their rebellion.

Now Julia does not worry so much about these things. She replies to Winston and says, “We’re not dead yet” and attempts to prove it with her body. But Winston feels that she is just too young to understand the true nature of their situation. He thinks to himself,

She would not accept it as a law of nature that the individual is always defeated.

Winston lacks the arguably naive confidence that Julia has in their ability to live a life hidden from the Party. And in the end, the reader ultimately sees he was right to think this way. Even though the Party does not kill them, they become loyal to the Party and lose the sense of individuality and privacy that they thought they had while rebelling together.

In 1984, Winston first says, "We are the dead" as he begins his affair with Julia. While they may have months or years of physical life remaining to them, their rebellion against the Party has put them beyond hope of living out their natural lives. Like thoughtcrime, their disobedience does not merely entail death, it is death. He also says that "death and life are the same thing." Julia thinks that this is nonsense, but it foreshadows the end of the novel, when the fate of both Winston and Julia is not physical death but a form of mental and spiritual death.

When Winston first says, "We are the dead," he and Julia are engaged in a purely personal rebellion against Party orthodoxy. However, the words gain new meaning when O'Brien says them at their meeting in his apartment. What Winston has now committed himself to is an active political revolution, which, even at this most hopeful stage, he does not believe he can actually win. For the rebels, to say that they are the dead is to resign all personal hopes for the future and to concentrate on doing what little they can to undermine the Party. Finally, both Winston and Julia say the words again just before their capture at the end of part 2, acknowledging that neither political rebellion nor personal idyll can last.

Although Winston Smith would dearly love nothing more than to bring an end to the Party's tyranny through his subversive activities, he's a realist and knows that he will not succeed in his endeavors. He is a dead man walking, and he knows it.

That's what he means when he says, “We are the dead.” And as the “we” in that statement would imply, he's not just referring to himself; he's also talking about Julia. She, too, is involved in subversive activities; she too is, to all intents and purposes, dead.

Winston may be committed to the destruction of the Party and its iron grip on society, but he's brutally realistic about his and Julia's chances of success. He's acutely aware that, one day, perhaps in the not-too-distant future, he and Julia will be found out by the authorities. And when their subversive activities have been revealed, it's likely that they will end up dead.

That being the case, it's better to regard yourself as already dead and hope against hope that, one day, their brave defiance of a cruel totalitarian regime, may just bear fruit and will stand as a shining example for others to follow. But for now, at least, there is no hope; Winston and Julia are effectively dead.

This is a very important statement in the novel.  When Winston states that "we are the dead," it follows a statement that speaks more to Winston's above assertion.  Winston insists that "She did not understand that there was no such thing as happiness, that the only victory lay in the far future, long after you were dead, that from the moment of declaring war on the Party it was better to think of yourself as a corpse."  On the surface of it, it would seem that Winston, having decided to take on the Party, has already come to see himself as a corpse - perhaps not immediately, but soon, and it would be permanent.  What efforts he and Julia make in their struggle against the Party will not bring about effect they themselves will enjoy.  It is for later generations to see the benefits of their works.

Furthermore, from early on in the novel Winston realizes that being convicted of thought crime is tantamount to a death sentence (or vaporization sentence, I suppose).  What he and Julia are contemplating will surely lead to their capture as thought criminals - which ultimately it does. 

When taken into a larger context, however, "we are the dead" could reference the death of the individual self.  That is, the death is not a physical death; it is a metaphorical death.  All that defines them as individuals will be taken away, as it is through the process of O'Brien's interrogation of them in the Ministry of Love.  In this sense, Winston's statement is a definite instance of foreshadowing.  Winston and Julia (and the rest of society) are the dead...and the Party has killed them, or at least their will to individuality.