Okay, I just figured something out... and considering DH has been out for nearly a year, I can't believe I just now put two and two together...
Anyway, I was rereading OotP and I came across this scene...

Chapter Two, A Peck of Owls, pg. 31-32(US hardcover)
"De-men-tors," Harry said slowly and clearly. "Two of them."
"And what the ruddy hell are dementors?" (Vernon)
"They guard the wizard prison, Azkaban," said Aunt Petunia.
.......
"How do you know that?" he(Harry) asked her, astonished.
Aunt Petunia looked quite appalled with herself. She glanced at Uncle Vernon in fearful apology, then lowered her hand slightly to reveal her horsey teeth.
"I heard - that awful boy- telling her about them - years ago," she said jerkily.

Okay, I for one always thought that she meant James... until DH came out. She was caught eavesdropping on the very conversation in which Snape was telling Lily about them.
Even so, it didn't really fit before that "that awful boy" was James, because Lily didn't start going out with him until their seventh year... surely she would have known about dementors long before then?
So was this scene some kind of foreshadowing? Something JKR just put in there to back up the Snape-Lily story in the earlier books? Both?
What do you think?