Arrietty is already up for preorder and has been for a few weeks. (down at the moment) However, Castle in the Sky and Whisper of the Heart just popped up on Amazon. They are currently listed as unavailible but the box art is in line with previous US releases of Ghibli movies on blu-ray...
This is "kinda" stale news.
Read the translated link here.
Read the actual link in Japanese here.
Basically, Akira, Ghost In The Shell and Steamboy on the BD+DVD set only.
fuad