UPDATE: The Hogwarts Professor (aka, John Granger) has inaugurated his blog with a huge post on "deathly hallows." As always, it's much deeper than you think it could be.
ORIGINAL: This is the title of the last Harry Potter novel due out (one hopes) next year. Of course, we all know that "hallow" is associated with being holy or being venerated (usually because of holiness), hence the phrase in the Lord's prayer "hallowed by Thy name." But the OED gives many definitions of "hallow." I think the relevant definition of "hallows" is the following: "In pl. applied to the shrines or relics of saints; the gods of the heathen or their shrines." In this case, though there has been speculation about graveyards (e.g., Harry's parents, or a graveyard at Hogwarts) as the hallows, I think it must be a reference to Voldemort's you-know-whats. Hence, the deathly hallows.
Or, Rowling is throwing us all off the scent and is thinking of another meaning of "hallow" in the OED: "The parts of the hare given to hounds as a reward or encouragement after a successful chase." Run away! Run away! It's the deathly rabbit parts! (No longer undetached, for you Quine readers.)
And, I don't think the reader ("Sirius") quoted in the news story linked to at the beginning of this post realizes what his worry reveals about his understanding of the Harry Potter series. He said, "This title has me a bit worried. For one thing, it has me concerned that Harry's gonna die . . . which I really DON'T want to see happen." Yes, death is a serious thing, even for a fictional character. But the worry that Harry will die indicates that the reader hasn't taken to heart a basic message of the books: there are things worse than physical death and if faced with a choice between death and doing right, one should do right with the assurance that love remains stronger than death.
I don't want to see Harry die either. But if it comes down to it, I'd rather see Harry die than not do the right thing. Perhaps the concerned reader, in his heart of hearts, thinks that, too, but is afraid. That's understandable, though we hear from reliable sources that perfect love casts out fear.
One last thing, if Harry does die, will that make the ending of the series better than the end of the Lord of the Rings with Frodo's "I do not choose to do this," etc.?