Simplest site to site bridged VPN; DHCP requests fail

I have two Linksys routers running Shibby (1.28).
Server: Basic = TAP, UDP, 1194, automatic, static key Advanced = defaults
Client: Basic = TAP, UDP, server address / port 1194, automatic, static key, server on same subnet
Advanced = defaults

I can't see that it could be any simpler: just bridge every packet between LANs.

But if I connect wirelessly from my MacBook to the Tomato client, the DHCP request fails.
If instead I assign a fixed IP address to MacBook, I can ping...

Simplest site to site bridged VPN; DHCP requests fail