I don't know if there's any special reasons why it's not available for amd64 yet. But it works for me on ~amd64.
Thanks for testing. The reason the amd64 keyword got dropped is a dependency on sqlite3-ruby, which is handled in bug 126540. sqlite3-ruby and rails will be keyworded in the same go, so i'll mark this one as a dupe of bug 126540 *** This bug has been marked as a duplicate of 126540 ***