Following Nathan's directions (thanks!) I got the card working just
fine. In case it is useful, I have documented the steps I went through
below. Of course, maybe changing the regdomain is not something you wish
to advocate in the FAQ..

# Steps to patch the regdomain from unknown (18) to FCC (16) for a D-Link
# DWL-AG530 so it works with current madwifi drivers.

1. Download the ar5k source, per Paul Shupak:
    http://sourceforge.net/mailarchive/message.php?msg_id=8966525

   Cut and paste the source into a file ar5k.c and then compile it:
    gcc ar5k.c -o ar5k

   NB: When I cut and paste, I get a typo near the end of the
   program; it reads ...don"t match..., should read ...don't match...
   Fix and recompile.

Note: The following step #2 is optional.  You'll need it only if you   
give an unknown value to your card.

2. Patch ath/if_ath.c, per Michael Gernoth:
    http://sourceforge.net/mailarchive/message.php?msg_id=9000820

   Rebuild the atheros modules, reinstall and reboot.

   NB: I had to reboot to single-user mode (i.e., LILO: linux single)
   because with an unknown regdomain, the patched atheros module rendered
   networking unusable (all networking, such as ifconfig, nfsd, samba,
   apache, would hang)

3. Make sure ath_pci is loaded:
    modprobe ath_pci

   Find its memory address:
    lspci -v > /tmp/lspci
    less /tmp/lspci

   Look for:
     0000:00:0a.0 Ethernet controller: Atheros Communications, Inc. AR5212 802.11abg NIC (rev 01)
        Subsystem: D-Link System Inc: Unknown device 3a14
        Flags: bus master, medium devsel, latency 168, IRQ 169
        Memory at ec100000 (32-bit, non-prefetchable) [size=64K]
        Capabilities: [44] Power Management version 2

    The memory address is on the second-last line of that section.

    Find the regdomain for your use; see Paul Shupak's mail; 0x10
    (16) is FCC.

    Run the ar5k utility:
      ./ar5k 0xec100000 0x10
             ^          ^
             |          +- The appropriate regdomain value
             +------------ From lspci

    It should print out that it has changed from 18 to 16, or whatever.

4. Reboot and everything magically works fine. You don't need to
   unpatch the atheros modules, but equally you don't need to repatch
   it in the future.

5. If you ever want to undo this change, just run ar5k 0xec100000 0x12
   (or whatever the original value was (0x12 == 18)).

-- Igor