Oooh, this totally made my morning. I love Sir Ian as Gandalf (particularly as Gandalf the Grey), and I really don't think the movie would be the same without him. I can't think of anyone I'd like to see in the role more.
Here's hoping Andy Serkis gets back on board, too. He owns the part of Gollum.
For those of you who are already becoming disappointed and disillusioned, don't lose heart yet! 2010 is a long way off, and we hardly know anything about the movie, yet. There is plenty of room for growth and change.
Edit: I see Del Toro is directing, so if nothing else, we know the movie will at least look pretty.