Still best practice is to have at least one of those servers for a setup with federated identities and Exchange rich-coexistence:
- ADFS server (plus ADFS proxy oder TMG or similar)
- DirSync server
- Exchange 2010 Hybrid server
btw: If you try to install DirSync 64bit on a Windows 2008 R2 DC you will see this message.
And it has still the same "scary" error about MSOLCoExistence registry key. Run the config wizard with elevated rights.
Error: Access to the registry key 'HKEY_LOCAL_MACHINE\Software\Microsoft\MSOLCoExistence' is denied.