Michael Lieberman points out Lenore Skenazy's column from the New York Sun last week, which is mainly about the buildup to the impending release of Harry Potter and the Deathly Hallows, but also notes a very interesting 19th-century example of a similar sort of literary anticipation.
"When Charles Dickens finished the last installment of The Old Curiosity Shop, in 1841, his American fans were so desperate to find out the ending that they stormed the New York piers and shouted to incoming ships, 'Is Little Nell alive?'"
Actually, as far as I can tell, the story has the crowds shouting "Is Little Nell dead?" The only mentions of the "alive" version seem to come from Skenazy and those who've quoted her column.* But be that as it may, it's still a fascinating parallel.
*If you find a contradictory example, please send it along.