I think they're pretty good dubs as far as dubbing anything goes, but I think that the original Japanese track is still better for most Miyazaki movies. The only real misfire, IMO, was the English dub for Porco Rosso, where Michael Keaton and Cary Elwes were both gravely miscast in their roles. In fact, I think the dub would have been better if they had switched -- at least Keaton could probably do a better cowboy accent. And, to be fair, Miley Cyrus did a more than credible job in Bolt, so fears that she'll get Hannah Montana cooties all over Ponyo seem misguided to me.
I can't really say that I'm all that impressed at the 800 screen rollout, though. True, it's more than any Miyazaki movie has ranked to date, but it's a pretty far cry from the 3,000-4,000 screens that a tentpole animated movie would do. Igor was a movie with no budget from an indie studio nobody had ever heard of, and it managed a debut on 2,300 screens. Heck, a colossal turkey like Delgo managed to end up on more than 2,000 screens. FWIW, Spirited Away got the widest US release of all of Miyazaki's movies, but that was still only 714 theaters. Most of his other movies were only on 300 or less.