I believe I have just found the answer.  I was told to set Jumbo Packets to 9000 for ant NICs that connected to the SAN network.  This seemed to interfere with the operation of the tape drive.  I set the Jumbo Packets to 1500 (default) and it is working great now!
I was running DPM 2010 that was connected to a 3TB RAID 5 array of SATA disks via iSCSI for our backup solution.  We recently upgraded the DPM server and and decided to use a 3TB slice of our PS4100 SAN as storage for the backups instead of the slow SATA array we were using.  I was very impressed to see the disk based backups now happen twice as fast but here is where it gets wierd.  Every night DPM takes the most recent disk backups (which are now residing on the SAN) and copies them to tape.  The tape backups now take about 10 to 20 times longer now that we are using the SAN as our backup storage.  Our File Server is about 500GB and used to take 3 hours to put onto tape.  It now takes about 20 - 30 hours to put on tape.  As a test I installed a 1TB SATA hard drive in a random workstation and made it an iSCSI target so DPM could connect to it and use it as storage.  I backed up our file server to this disk and then ran the tape backup and it took 2.5 hours to do 500GB. What is going on here?  Why is it 10 to 20 times slower to backup from the SAN to tape than it is to backup from an iSCSI target to tape?
Backup from PS4100 to tape is very slow