I've heard that just about everyone ever involved in Harry Potter has been invited to go to the grand opening. Although the main three haven't said anything, count on at least one of them being there (probably all three) also count on JKR and several of the other actors.
But don't count on autographs. Although some of the actors, especially the kids, may walk down the rows of fans and sign stuff for a few people in the front row, (like they do at premieres) it's doubtful there will be a "signing" with any of the main actors (Tom or the Twins or others who do fan events might be willing, but I wouldn't hold my breath for the big three). Also, I'd be very very surprised if JKR signed anything (I've dealt with her handlers before, it just doesn't happen unless it's been choreographed perfectly).
There's a part of me that really wants to go to the grand opening to see them all. But the other part of me, the meanie realist part hehe, says that going may be a bit disappointing. It's unlikely fans will get to ride the rides while the celebs are in that part of the park. WWoHP will likely be closed to us Mudbloods while the press gets its official taste of Butterbeer along with the celebs. And really, what's the point of going on the busiest day of the year if a girl isn't going to get the chance to make out with Alan Rickman on a dark and scary ride.


hehe