Certificate based authentication is a new great feature included in DPM 2012 and it will really help a lot of those people having a challenge backing up their production servers within an untrusted domain.
The one an only Shane Brasher MSFT at Microsoft published a series of three parts covering trouble shooting Certificate Based Authentication. Here are the URL’s:DPM Certificate Troubleshooting-Part 1: General Troubleshooting
DPM Certificate Troubleshooting- Part 2: Registry
DPM Certificate Troubleshooting-Part 3-Certificates
Shane your doing an amazing job!!