Anyone with experience running routed(8) in MacOS-X
care to explain if Apple have done anything that makes
it different from the BSD man page version?

Errors in /var/log/system.log claim that
arp-resolve fails because the upstream(default) gateway
is not in the local net.

but netstat -anr and arp -an both show correct
route to the correct hardware, and all applications run
correctly via en0 the main interface.

A network on the second NIC en1 has its packets passed
out thru en0, but the Macintosh does not acknowledge
arp requests for the host which sent the packet (and
is waiting for a reply). netstat shows a route exists
to that host.

It's almost as if routed and the OS-X kernel are
maintaining separate routing tables, and not talking
to each other....