It's hard to say, but I'd say that the Harry Potter series has a good amount of strikes going against it as far as what statistics show end up being classics.
1. Fantasy books are very rarely considered classics. Why? Well as someone's already pointed out (was it TK?) in the other forum, they're very self-referential, and that makes it hard for them to be socially relevant. Ironically, people often state that clasics are timeless, but a majority of the time the books quickest to become classics are the ones that most closely reflect the author's relevant events or philosophies. That's not to say fantasies can't become classics, and some people consider a lot of books geared towards children to be acceptable fantasy classics (CS Lewis, Lloyd Alexander, Susan Cooper). Although I think the adult and general fantasy is slowly becoming accepted as well (JRR Tolkien and TH White for example).
2. As a lot of people have already said, it's very popular right now, which grants it more of a pop-culture status. Academia is becoming more and more close-minded. The interesting part about it is that a hundred years ago, academia avoided folk stories and so forth, and now folk stories are more accepted but if something has any scent of commercialism (movie versions, merchandizing (yeah that's right, it's a z you crazy Brits)). It's not that the ivory tower folks have to lecture using JK Rowling as a topic for 30 years for it to be considered a classic, really. So if academia doesn't like it, I think that enough people can claim it as a classic, especially if they pass it down to their kids.
3. Aside from her ability as a raconteur, she doesn't display a lot of technical prowess in her style. What's additionally relevant lately is that as Thanatos pointed out, her intricate plotlines and complex character correlations are being destroyed by her own strict control over the interpretation of the characters. As a great novelist (was it JD Salinger?) once said, "My work is done as soon as I've signed off on the final draft. If I have to make any changes or explanations off the page, I failed on the page."
4. Not only did she publish the book geared specifically towards children, she published them in a fantasy world, completely of her own design and very strictly determined by way of her contracts (in film for example) that it was for a British audience. While this may or may not affect the way we view it in twenty years, narrowing the appeal of a book can't be helpful on the world stage. Obviously, this book has exploded worldwide, but different countries think about in very different ways because of the target audience of the original version.
That's about enough ranting for me. I don't know how I feel about it becoming a classic. If it does or doesn't, we likely will be too old to care by the time it is declared as such. I think of them more along the lines of other fantasy novels, like Dragonlance or the Wheel of Time series. It's well-written and fun, but if it never evolves beyond the realm of subculture it wouldn't be a crime.
