Geoff,
We have server to server communications between two TSM servers in
data centers about 60 miles apart.  A db backup is written to a
virtual volume at the distant data center. That has eliminated moving
cartridges by courier.   There is a  private 10 Gb link  between the
servers.   The servers are on HP DL585-G2 boxes with 4 dual-core CPUs
running RHEL  4.   An aside: until recently they had 16 GB RAM.  When
that was upgraded to 40 GB it made a significant  improvement in
client backup throughput, but not much difference in disk pool backups
and migrations.

Best wishes,
Keith Arbogast