Abebooks.com, a large online seller of rare books, reports that they surveyed 608 Harry Potter fans and found 20% expect to learn of the ending to book 7 from leaks in advance of its publication:
Editor's Note: No doubt we'll all have to practice constant vigilence to shield ourselves from leaks this last time around. Anybody who thinks there simply won't be any leaks has been woodwinked by the Confundus Charm, unfortunately. Even with the tightest security in publishing history, details will undoubtedly find their way to light before official publication.
HPANA is already making detailed plans to shield visitors from spoilers, based on our experience last year.