It's not the first time the two have come together for a potential project. Spielberg has been trying to bring King's 1984 novel (with Peter Straub), The Talisman, to either the big or small screen for roughly 20 years. TNT came close to putting together a six-hour mini-series in 2007, but the TV event (as I believe they call these things) was shut down due to budgetary reasons. Imdb.com currently has a listing for a 2010 version of The Talisman series, but there doesn't seem to be news of such an endeavor anywhere else.
As for Under The Dome -- a quick visit over to the book's Wikipedia page reveals a wealth of positive reviews, including the likes of the Associated Press, The New York Times and the Los Angeles Times. Consensus seems to be that this is one of King's best in quite some time.
In a YouTube clip from a Nov. 10 book signing in Maryland, King states he thinks the series will "actually" go to HBO, which would make sense because that's where most interesting (and expensive) TV productions go. A brief, spoiler-free glance at the synopsis reveals exactly what you would expect from the book cover -- a small town becomes sealed off to the outside world when a mysterious dome of sorts gets dropped down on top of it. Chaos ensues amongst the townspeople.
So without any further ado, let me be the first to add the cry of, "Simpsons did it! Simpsons did it!" (Actually, King has been working on some version of this story since the late 70s, early 80s, well before The Simpsons Movie.)