I know that there has been some confusion concerning the publication of DARKE, but find below the official dates confirmed by HarperCollins and Bloomsbury.
USA HarperCollins: Hardback Edition, Ist June 2011
UK Bloomsbury: Export Edition (paperback edition for Australia, South Africa and other Commonwealth countries), 6th June 2011. This will not be for sale in the UK itself.
UK Hardback Edition: October 2011.