Disney today released a promo clip for Arrietty, in anticipation of next month's theatrical release. This is a short clip, but it should give you a good idea of what to expect from the Disney dub. Oh, and be sure to pay close attention - there's a big surprise at the end. Heh heh.
I expect that Disney will release a few more clips online; when that happens, we'll be sure to post them here on The Ghibli Blog. Feel free to share your impressions here.