Interesting. Some of the things that have been described as silver or silvery so far:

• The Patronus charm (particularly the True Patronus)
• The Deathly Hallows symbol in this chapter
• The stars in space
• The Invisibility Cloak (in canon, at least)

All of these seem to have in common that they represent some sort of resistance to death or indifference (usually represented by coldness, like the vacuum of space or Harry's dark side). This has probably already been pointed out a lot, but I predict that whatever is glinting silver in the prologue represents something similar, even if it's something else entirely (e.g. a dagger, the Sword of Gryffindor, etc.)

Edit: also, as someone pointed out earlier, the Philosopher's Stone now turns metals into silver as well as gold (see Hedonic Awareness).

I think "likely" may be an overstatement at this juncture. The entire Deathly Hallows insignia hardly seems like "a tiny fragment… a fraction of a line". I suppose it's possible that some ritual results in the glowy part being erased until only a small portion of the wand is left. But the word "glint" sounds like it's a metallic object moving and flashing light briefly, not something glowing with its own light continually for a time. And while it's possible that Harry will be driven to spill liters of blood to resurrect Hermione, that sounds more like a ritual for Little Hangleton than for Godric's Hollow.

It's definitely a candidate, though. It's in a graveyard, which as we all know is a great destination for bloody moonlit rituals in Harry Potter books. There is a silver line involved. Too early for "likely", but worth keeping in mind.

If so, then:

• Presumably what's happening is some ritual involving the Deathly Hallows, carried out by Harry or Voldemort or both
• Most likely with the goal of defeating Death (somehow)
• So I guess the litres of blood are a requirement of the ritual (note: in view of earlier stuff about how modest the things sacrificed in rituals are, it had better be something as grand as defeating Death)
• Harry doesn't seem like the type to spill other people's blood even for such a goal (though, I dunno, he might) and Voldemort may quite possibly wish to spill Harry's
• So maybe it's Harry's blood, and he's (directly or, or letting Voldemort do it) sacrificing his own life to put a permanent end to death for everyone else
• Which would make a certain amount of dramatic sense
• In which case, probably the word being screamed is "No!" or "Harry!" or something of the kind
• Though maybe it ends up with him getting resurrected too; see, e.g., canon Harry Potter, Narnia, Jesus -- the first two being deliberately derived from the latter; not so congenial an idea to Eliezer as to Rowling (who is a Christian), but note Eliezer's "Ta-da" remark when Leah mentioned "Christus Victor" and divide-by-zero errors
• So maybe e.g. Harry needs to be killed while invisible to Death because of the Cloak of Invisibility; perhaps that sends his soul-or-similar-thing to wherever wizards' souls go, but without him actually being properly dead, and enables him to fix things up there (a very "Christus Victor" idea; maybe too much so)
• None of this seems to connect with the prophecy about "the one with power to vanquish the Dark Lord", but maybe as others have speculated the Dark Lord in question is Death or something

All, of course, pure wild ass-speculation.

I always figured that was a knife, flashing. Y'know, because of all the blood.

The black robes might be a Dementors cloak. The falling might mean the Dementor moving faster than any broomstick, the robes being left behind.

Harry resurrecting his parents?