Exchange 2013 Preview has been released and can be downloaded from the following URL:
http://technet.microsoft.com/en-us/evalcenter/hh973395.aspx
Be aware that it is only available as a 64bit version and can be installed on Windows Server 2008 R2 and Windows Server 2012. The following URL will give you all of the prerequisites required to install Exchange 2013:
http://technet.microsoft.com/library/aa996719(EXCHG.150).aspx
Enjoy!
Cheers
Paul