>> On my PB550C, with the kernel available at
>> ftp://ftp.aa.ap.titech.ac.jp/pub/yanagisw/NetBSD/netbsd_PB550c_SONIC.
>> tar.gz, done by Takeshi Yanagisawa, my PB can find the 550c's MAC address
>> if I boot with extensions, especially the Onboard ethernet extension
>> loaded.
> Yeah.  That code, and my first cuts at the SONIC driver, loaded the MAC
> address from the SONIC.  The newer, checked-in, code reads it from the
> PROM and sets up the SONIC with that info.

  More specifically, the original code expected the MAC address
to be the first location in the Sonic's CAM. The MacOS will place
the ethernet address in the CAM if something has used the ethernet:
if you had AppleTalk turned on or you ran a TCP/IP app which
caused OpenTransport to load the ethernet driver.
  If nothing in the MacOS wanted to use the ethernet, the Sonic
will not get initialized so we can't get the address that way.
The -current driver reads the ethernet PROM instead, but the PB500
PROM is not the same as the Quadras.

>                                             Unfortunately, it appears
> that Apple stores the Ethernet PROM in a different location on the
> PowerBooks.

  I suspect that furthermore it is a serial PROM.
The description of the Whitney IC in the PB500 DevNote
implies it is doing more than generating a chip select
for a byte-wide PROM.