rdiff-backup --remote-schema
  'cstream -v 1 -t 10000 | ssh %s '\''rdiff-backup --server'\'' | cstream -t 20000'
  'netbak@foo.bar.com::/mnt/backup' localbakdir

(must run from a bsh-type shell, not a csh type)

That would apply a limit in both directions [10000 bytes/sec outgoing,
20000 bytes/sec incoming].  I don't think you'd ever really want to do
this though as really you just want to limit it in one direction.
Also, note how I only -v 1 in one direction.  You probably don't want
to output stats for both directions as it will confuse whatever script
you have parsing the output.  I guess it wouldn't hurt for manual runs
however.