The MDT 2013 Preview was made available the other day on the Connect website. You can download it here: https://connect.microsoft.com/ConfigurationManagervnext/Downloads/DownloadDetails.aspx?DownloadID=50082
You'll also need the ADK for Windows 8.1 Preview, which you can get here: http://www.microsoft.com/en-us/download/details.aspx?id=39306
David Coulter | http://DCtheGeek.blogspot.com |@DCtheGeek