As far as I can tell, DPM 2010 only supports Short Term To Tape Incremental backups (and at that, only if you choose "Daily" instead of "Weekly" as the option). When using Long Term to Tape, only Full Backups are performed.
Does anyone know if DPM 2012 will support Incremental backups for Long Term to Tape? This is sorely needed by us in our situation, and we would love to see this feature added.