A fan of The Leaky Cauldron has let them know that Amazon has published the length of Harry Potter and the Half-Blood Prince library binding version as having 672 pages. Barnes & Noble have also listed the same page count.
It is possible that this is the real page count for Half-Blood Prince as the hard copy and library binding versions of Harry Potter and the Order of the Phoenix had the same published number of pages.
As of now there has been no information published about the length of the book other than it will be less than 38 chapters and that if there is any news published the correct place to read about it will be at JK Rowling's site.
UPDATE: Thanks to everyone who let us know that Barnes & Noble have also posted a deluxe version of Half-Blood Prince which contain have 704 pages (32 more pages than the hardcover and library binding editions).