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!