Borders bookstores have said that pre-orders for 'Half-Blood Prince' should be made before July 2nd. This is due to the fact that huge demand for the book is threatening to outstrip their supply. Anyone who orders the novel after that date will be put on a waiting list and if a book becomes available, they'll receive 30% off the selling price.

Additionally pictures of many boxes storing copies of 'Half-Blood Prince' can be seen here courtesy of a Borders employee.

Source: The Leaky Cauldron