Dumb dumb dumb... I was using the second NIC on three linux boxes to test with - 192.168.3.253,252,251 and the ASA5505 was at 192.168.3.1. Pings to 192.168.3.1 from the vpn client worked, the others did not.
If you look at the vpnpool in the above config, you will discover that clients come in at 172.16.1.100-199... and there was no route for that on the linux boxes. The default route remained on the production firewall via the primary NIC. Adding a route for 172.16.1.0 to 192.168.3.1 fixed things.
That error message on the client side still bothers me. I'd like to know what the problem is.