You know what this is going to be about...
These are the results dictionary.com gives for "deathly":
-adjective
1. causing death; deadly; fatal.
2. like death: a deathly silence.
3. of, pertaining to, or indicating death; morbid: a deathly odor from the sepulcher.
-adverb
4. in the manner of death.
5. very; utterly: deathly afraid.
[Origin: bef. 1000; ME dethlich, OE déathlīc. See death, -ly]
These are the things that come up when one enters "hallows":
tr.v. hal·lowed, hal·low·ing, hal·lows
To make or set apart as holy.
To respect or honor greatly; revere.
[Middle English halwen, from Old English hālgian; see kailo- in Indo-European roots.]
So--the first adjectival definition mentions "deadly" as a synonym. That often means that the synonym is the preferred form. These are the definitions that come up for deadly:
adjective
1. causing or tending to cause death; fatal; lethal: a deadly poison.
2. aiming to kill or destroy; implacable: a deadly enemy.
3. like death: a deadly pallor.
4. excruciatingly boring: The dinner party was absolutely deadly.
5. excessive; inordinate: deadly haste.
6. extremely accurate: Annie Oakley was a deadly shot.
-adverb
7. in a manner resembling or suggesting death: deadly pale.
8. excessively; completely: deadly dull.
[Origin: bef. 900; ME deedli(ch), OE déadlīce. See dead, -ly]
You'll notice that "deathly" does not appear as a synonym for in any of the definitions for "deadly". The word usually used for something that causes death is "deadly". You even see this in plant names. It's "deadly nightshade", not "deathly nightshade". The adjectival use I see most often for "deathly" is the second one: like death ("a deathly silence"). So we already have a problem: the structure of the title is "Harry Potter and the Adjectival Noun", so choosing one of the adverbial definitions is not really appropriate in this case, and "deathly" is rarely the appropriate adjective for any noun. (Other than "silence", "danger" comes to mind as a possible word that can be appropriately modified by "deathly", but I still think "deadly" works better there.)
There are no definitions of "hallows" on this particular site that make it a noun. None. It is a conjugation of a verb, not a plural noun, when properly used in modern English. Now, it is a regular use of "hallows" to include it in the phrase "All Hallows Eve", which is the evening before All Saints Day. It is not generally referred to as "All Hallows Day", although there are numerous churches and schools called "All Hallows". It is still a verb in this usage (being used as an adjective), and just a shortened form of a phrase referring to all being hallowed, sanctified or made holy. The holiday is a celebration of all of the saints--all who are holy, or hallowed. It is not really being used as a noun in "All Hallows".
In the title it's being used a little like "holies" in the phrase "the holy of holies", but it's really not appropriate to use an adjective with "holies", either, due to the fact that "holies" is already sort of modifying itself--it means "holy things". So you could "translate" the title into clearer English as "Harry Potter and the Deadly Holy Things" or relics. Which could, in turn, mean the Horcruxes, but damn if that isn't any better than "Deathly Hallows", apart from being a little (but ONLY a little) clearer in its meaning. (And it's interesting that this doesn't even show up as an archaic usage in online dictionaries). My old two-volume OED (so it's not a complete one, I'm afraid) may have a noun definition of "hallows", I suppose. If so, this is such an archaic usage, I think JKR'S publishers might prefer her to use the word "philosopher" again!
Oh well. We've already seen that titles are not her forte. "Harry Potter and the Half-Blood Prince" was pretty good, as was "Harry Potter and the Order of the Phoenix" (the best of the seven titles, IMO, unless she comes to her sense and changes HPATDH to something even better than OotP). Before that, the best title was "Harry Potter and the Chamber of Secrets". The title of the first book was bad in either the UK or US versions, for different reasons; in the UK version it was telegraphing the importance of alchemy (mentioned on Dumbledore's Famous Wizard Card) way too soon; and the US version was an awful marketing decision that instead made alchemy absolutely irrelevant to the book, in that there is no lore connected to something called a Sorcerer's Stone. (When I realized, in the course of my reading, that the "Sorcerer's Stone" was really a Philosopher's Stone I felt like throwing the book across the room, since I know what a freaking Philosopher's Stone is.) The third title was sort of "eh" and not terrifically accurate, since Sirius was an ESCAPED prisoner. The fourth book's title had very little to do with the book but was admittedly better than "Harry Potter and the Doomspell Tournament". The fact that she even CONSIDERED that title should tell us something: lighting on "The Order of the Phoenix" was sheer luck. :D
It's possible that what JKR means by "the Deathly Hallows" is the night that Harry's parents were killed (it was certainly a DEADLY Halloween for them, or ALL HALLOWS EVE). Which, I think, makes it as disconnected from what is likely to be happening in the book as the Goblet of Fire was to the plot of the fourth book. Yes, that night was a major turning point, the night that Harry Potter was really made into "The Boy Who Lived". But it's very problematic grammatically if that's the meaning, rather than "holy things". I'm just hoping that she's been so focused on the CONTENT of the book that she hasn't had that much time to give a lot of cogent thought to the title. Because as titles go, it's probably the worst of the seven. Frankly, I totally understand those people who thought it was "hollows" instead of "hallows". At least "hollows" is a plural noun in modern English! I predict that "hallows" is going to be even more abused and mangled between now and the book release than "Horcrux" has been since HBP.
And the fact that "deathly" also has an adverbial meaning makes me wonder whether she's thumbing her nose at all of the critics complaining about her Tom Swifties and general abuse of adverbs. If that's the case, I'm somewhat amused, but I still wish she had chosen a title that's clear, grammatical modern English, in which adjectives are adjectives, nouns are nouns, and the different parts of speech aren't trying to muscle in on each other's territory. It would also be nice if one of the major words used hadn't gone out of fashion during the Middle Ages. ;) (I think I'd prefer a word she'd made up to be in the title, like "Azkaban" or "Horcrux".)