World's Greatest Predictions

1Hallows = Horcruxes	2Privet Drive to Grimmauld Place	3The Identity of R.A.B.	4Snape's Loyalty & Patronus	5New Leadership: Lupin and Krum
6Wormtail the im-Percy-nator	7All about Godric's Hollow	8St. Mungo's & the Longbottoms	9Werewolf Games	10Harry the Horcrux
11Death and Romance	12Lily's Eyes, Harry's Eyes	13Ginnymented: the Final Battle	%Complete List with Probability	&Leftovers

Deathly Hallows are Horcruxes

What is a "Hallow"? Stumbleine_ (from HPANA) already had an answer last April:

The Holy Grail is just one of the Hallows kept at the Grail castle. Hufflepuff’s cup is just one of the Horcruxes kept at Hogwarts.

The Hallows are said to be:

The Holy Grail

A sword

A platter (which often bears a head)

A spear

Voldemort's Horcruxes were originally meant to include something from each of the Founders, hence we can guess the two rejected titles for book seven: Hogwarts Hallows and Hallows of Hogwarts.

That helps us to identify the remaining Horcruxes, which is handy, but any multi-soul-splitting Dark Lord can tell you the three most important things in the Horcrux business are location, location, and location.

There could be a Horcrux hidden at Hogwarts. If not, expect Voldemort to want one there, which means he'll be taking his war to the school:

"Reckon Dumbledore's the only one You-Know-Who was afraid of. Didn't dare try takin' the school, not jus' then, anyway."
(Hagrid, PS 4 pg 55)

With Dumbledore dead, the way is clear.

(credits and citations)

Hallows, Tarot and the Elements

Various theories have connected the Horcruxes to the four suits of a tarot deck: cups, swords, wands and pentacles. The tarot connection is supported by names like "The Hanged Man" (a pub in Little Hangleton) and "The Lightning-struck Tower" (HBP 27), both Major Arcana from tarot.

Both the standard tarot suits and the Grail Hallows are connected to the four classical elements: earth, air, water and fire. Rowling herself said the four Houses of Hogwarts correspond to these elements.

All of these patterns have a "gap" in the known Horcruxes which can be filled by a wand (corresponding to the element of fire). Hairy Pooter (also on HPANA) was discussing the classical elements when he observed this curious passage:

The last shop was narrow and shabby. Peeling gold letters over the door read Ollivanders: Makers of fine wands since 382 B.C. A single wand lay on a faded purple cushion in the dusty window...

...For some reason, the back of his neck prickled. The very dust and silence in here seemed to tingle with some secret magic.
(PS ch 5)

The "something of Ravenclaw's" which Voldemort made into a Horcrux could be this very wand. It would help explain Ollivander's disappearance at the beginning of Half-blood Prince.

(credits and citations)

Dedication
Flipping past the title page, there is usually a dedication. Who else but Rowling's mother, Anne Volant Rowling?