If you are getting error:
"An error occurred. The error code was 3221685466. The message was The service is already registered.." while trying to uninstall Exchange 2007 SP1 Mailbox role from Server 2008 R2
Try changing registry key value from "Uninstall" to "Install" located at
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Exchange\v8.0\Mailbox\
after that try uninstalling again.
Your mileage may vary -- so far it's worked 5 out of 6 tries for us.
