For those who asked, the major actors (Daniel Radcliffe, Rupert Grint, Emma Watson, Tom Felton, and a few others) are contracted to appear in the first four Harry Potter movies--in other words, through at least Goblet of Fire. There has been no confirmation that they will star in any of the rest of the movies. Goblet of Fire is currently in the early stages of filming and in the next few weeks we should hear a lot of announcements about who is starring in such roles as Fleur Delacour, Viktor Krum, etc. They do have a release date scheduled for Order of the Phoenix, but that's very, very far away in terms of even beginning to nail down casting. I think that they will try to retain most of the actors, but you can bet that the kids are going to negotitate for more money. I know there's been talking of the actor who plays Hagrid leaving if he doesn't get more money.
As far as the sixth book, there have been crazy rumors saying it's coming out in September... I don't think we'll see it until at least summer 2005, more likely summer 2006.
Again, the best three news websites (I love them, have to plug them!):
www.the-leaky-cauldron.org
www.hpana.com
www.mugglenet.com