WiFi <=> Ethernet bridge not working; advice?

[Mark Shroyer <subscriber+netbsd%markshroyer.com@localhost>]

I'm trying to turn a bunch of old laptops into 802.11b to Ethernet
bridges.  For some reason, while I can bring up both the wireless and
the Ethernet interfaces on these laptops and add them to a bridge,
NetBSD (5.0.1) doesn't appear to forward frames across the bridge for
me.

I've tried this on two different laptop models with different wireless
interface drivers (one wi, one ath).  I figure I'm probably just making
some dumb mistake here, so any advice would be appreciated.

Here's what I did:

 1. Bridge the interfaces together:

    # ifconfig bridge0 create
    # brconfig bridge0 add wi0 add ex0 up

 2. Bring up the wired interface:

    # ifconfig ex0 up

 3. Bring up the wireless interface and get a DHCP assignment:
    
    # ifconfig wi0 nwid TestNetwork up
    # dhcpcd wi0

    At this point I am able to communicate with the test wireless
    network from the bridge laptop.

 4. Plug another computer (let's call it the "client laptop") into the
    bridge laptop's Ethernet port with a crossover cable.  Observe that
    the client laptop fails to negotiate a lease with the DHCP server on
    the test wireless network.  Assign a static IP address to client
    laptop's Ethernet adapter and attempt to ping a computer on the test
    network; observe that client laptop cannot ping the test network,
    however it can ping the address of the bridge itself.

I'm out of ideas.  Brconfig shows that the bridge is up and correctly
learning MAC addresses on both sides, but for some reason no traffic
gets across.  Thanks in advance for any suggestions...

-- 
Mark Shroyer
http://markshroyer.com/contact/