We also would do only a tape-backup from the R100, no local tape drives in the remote offices.
Phil,
I recommend you get an R100 loaner from Netapp and attach your tape drives. Depending on your drives, the R100 will not be able to properly keep your drives running at full speed.
For example, AIT-3 drives connected to host running NFS to R100 or F880 can do 20+MB/sec. AIT-3 drives connected to R100 or F880 directly can only do 10+MB/sec. Also, watch CPU utilization when you run NDMP locally vs. backups over NFS. Each drive will use 30% of R100 CPU, but only 15% of F880 CPU (even though the CPUs are the same).
You truly need to test this in your environment before making purchases. If you're backing up terabytes of data from remote offices, get a true F880c or F960c instead of R100. If you have a standby filer sitting there, you could move the shelves from the snapmirror destination filer to the standby filer with no data copying. With the R100, this is impossible.
/Brian/