If you are encrypting DPM tape based recovery points, you should also export and maintan a copy of the encryption certificate located in the Local Computer Certificate store under the DPMBACKUPSTORE.
See the following links:
How to Import Certificates into DPMBackupStore
http://technet.microsoft.com/en-us/library/bb808937.aspx
Manage certificates for your user account
http://technet.microsoft.com/en-us/library/cc780916(WS.10).aspx
Alternatly, If the certificate was created and imported from a CA, make sure the originating CA and certificate is still available. The certificate is also backed up as part of systemstate backup, so maintain a remote copy of systemstate backup so you can recover shoukld you have a DPM server loss.