Well, I learned a quick lesson today about having new Exchange 2013 servers in my current 2010 site.

1.  Install the Mailbox server first (if you plan on separate CAS/MBOX roles.)

2.  To access the new control panel, use the server name and the version: https://yourservername/ecp?ExchClientVer=15

3.  Make sure to add Exchange Trusted Subsystem to the Administrators group.  This one drove me crazy until I realized that I had the restricted groups setup via GPO.  Until you do this, expect to get IIS errors in the ECP on your new servers, stating :

An IIS directory entry couldn’t be created. The error message is Access is denied.

HResult = -2147024891

This apparently happened when running 2007 and introducing 2010 to the mix.  Remember to either gpupdate or preferably restart the 2013 servers once you add that entry!