When designing a tape backup solution using DPM you should keep these concepts in mind:
1. Make sure the tape drive and library are part of the MS support list. http://technet.microsoft.com/en-us/dpm/cc678583.aspx
2. DPM supports iSCSI tape drives.
3. By default, if you don't change the tape or if you don't have enough space in the tape, the DPM backup process will fail in 60 minutes. The default timeout value is 60 minutes. http://santhoshsivarajan.blogspot.com/2009/10/dpm-2007-and-tape-timeout.html
4. By default, DPM has a 20% “free tape” threshold and is not configurable.
5. DPM does not support static labels. DPM uses an on-media identifier (OMID) to identify tapes. The cleaning tapes barcode starts with “CLN”. http://technet.microsoft.com/en-us/library/bb808923.aspx
6. DPM does not support Write Once Read Many (WORM) tapes.
7. For backup, DPM will start reserving from the highest numbered slot backwards.
8. You can group recovery points of multiple protection groups on a single tape by enabling the Co-Location feature. http://technet.microsoft.com/en-us/library/cc964296.aspx
9. DPM supports encrypting data on tape for long-term protection. http://technet.microsoft.com/en-us/library/bb808967.aspx
10. You can use tape for both short-term and long-term protection of file and application data.
11. DPM supports compressing data in protection groups on tape for long-term and short-term protection. DPM does not support compression if encryption is enabled in Protection Group.
12. In DPM, you can back up data from a computer directly to tape.
13. If your DPM server is installed on a Hyper-V server as a guest, It supports only disk backups. You cannot enable pass-through configuration for the tape library.
14. Make sure to select appropriate capacity tapes/drives
Compressed Capacity (2:1)
Also, here is a good article on “How DPM Uses Tape Libraries”- http://technet.microsoft.com/en-us/library/ff653494.aspx