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).