I think Speilberg is a very interesting artist because his work has changed much more than say Hitchcock or Scorcesse ever did (not to say that Al and Marty never changed, there are big differences between Rebecca and Frenzy and differences between Mean Streets and The Aviator, however they have the same mark of the same artist).
I think Speilberg's mass audience work has taken a dip in this decade, as I think many will agree that WOTW and AI are no Jaws, Close Encounters, Indiana Jones or Jurassic Park. However, I agree with Kevin that his work has gotten darker.
He may have been more ballsy in his youth, but he has become more mature with time. I think it actually started with Empire of the Sun. This was a very heartfelt movie set during WWII (his most vested time period, really) and was very moving. He reached his high point as an artist with Schindler's List. This was when he felt he reached the point to try something like this. Since then his movies had a darker tone (compare The Lost World to Jurassic Park and AI to Hook or ET), but I think the real change in his filmmaking making it more cynical was 9/11.
I think 9/11 had a huge impact on Speilberg because before it, his darkest work was still optomistic and even hopeful (the ending of Schindler's List is a very moving one, with the silver lining in the cloud). Take Amistad which tackles the depressing issue of slavery, it is still a very uplifting movie even if the last note is remorseful (as much of the film is). AI is bittersweet.
Yet, the only real movie that I would say is "happy" since 9/11 is the cat and mouse Catch Me if You Can (which I consider one of his more underrated movies at is immensly entertaining and well made/acted).
But Minority Report is far more condescending of the world we live in and the corruption that surrounds it. Minority Report ends "happily" but not hopeful and is far removed from Raiders of the Lost Ark. War of the Worlds evokes imagery of the Holocaust but also a lot of 9/11. I feel that someone twisted their arm into that ending (as there is an alternate ending out there somewhere Speilber mentioned, hopefully with a dead Robbie) to make the Tom Cruise summer movie happy in its conclusion. However, the whole movie is the grimmest look at life one can have and far removed from Close Encounters. How many summer movies has the hero beat to death a fellow survivor who saved the hero's life earlier while his daughter is in the next room?
Munich is a film I think that tackles the issues Speilberg has been wrestling with since 9/11 and ends far more pessmistically and much more ambiguously than any film he has ever made. Because there is no answer. While just as violent and gut wrenching in the superior Schindler's List and Saving Private Ryan....he was hopeful in both and gave a better view of the overall world and humanity. That view is almost entirely absent in Munich.
It has been fascinating that as his crowd pleasers have gone down hill (though when he can still produce Minority Report and Catch Me if You Can and the action scenes of WOTW, I'd hardly say he has lost his brilliance in that department) his more intimate projects have grown more complex and darker (including his opinion of the world it would seem).
I think after his time has ended Speilberg will be much more appreciated as an artist than he is now (many say he relies on computers and puppets, which is complete bull **** in my opinion).