Okay, my routing wasn't working because the machines on the subnet 
don't know what the default gateway is supposed to be (I finally get it 
- thanks Fred Wright - I was overlooking the obvious).

In my current, and hopefully soon replaced, setup on my Linux DHCP 
server provides the default gateway address via the "option routers 
192.168.15.1" command in /etc/dhcpd.conf.

I don't see where, in m0n0wall under the DHCP pages, there is a place 
to provide the default gateway.  I'd like the machines to be able to 
move between the subnets seamlessly, but that would seen to require the 
gateway be served via DHCP.  Is there a way to do this?

Greg