From today on Microsoft Websites you can download Microsoft Exchange Server 2013 RTM evaluation version.
This version can be installed and it’s working for 180 days.
This allow us to get to know Exchange Server 2013 RTM in test or lab environments.
Exchange 2013 RTM works on Windows Server 2008R2 and Windows Server 2012.
Remember that coexistence Exchange 2010 and Exchange 2013 is not supported.
It will be probably supported when Microsoft will release SP3 for Microsoft Exchange 2010.
Below you can find useful links for Exchange 2013 RTM:
Download Microsoft Exchange Server 2013
Exchange 2013 System Requirements
Microsoft Exchange 2013 RTM Documentation
English 
polski