When Xenoblade was released in Europe, it had bad reviews about its English voice acting. It was enough for some people to use Japanese voice acting with English subtitles. From the voices of The Last Story's trailer, the voice acting seems to be excellent so far actually. It does make sense to use american voices in the american version of the game but the game will also be released a day before in Australia so the american version won't probably have american voice actors. There's no need to use english subtitles when the voices are in english...
Note: Updated topic with news pictures to show the graphics and gameplay. The pictures does not exactly show the real graphics because pictures are resized and converted into different formats therefore reducing picture quality.