The last two Harry Potter films are going to be fighting an uphill battle, what with the rest of the tweeny-bopper world moving on to be Twi-hards. We've had two Twilight movies now, surrounding The Half-Blood Prince like some horrible sandwich. Couple that lessening familiarity with the unmistakably dark tone of the last book and the ever-increasing ages of the lead actors and it's not going to be too easy to get kids excited about it.
Mokalus of Borg
PS - I say "the last two films" because I heard that's what's being done.
PPS - Plans may have changed since then.