As you may know, DPM 2007 adds support for tape to DPM 2006's disk-based protection. So with DPM 2007, you can configure protection disk-to-disk, disk-to-tape, or disk-to-disk-to-tape. But an important question you'll have is, will DPM work with my existing tape library? Or, what tape library should I get to work with DPM?
For your answer, take a look at the list of tape libraries that have been tested and found compatible with DPM so far. Keep in mind that just because a tape library is not listed there, it does not mean that it is not compatible--only that it has not yet been tested.