Thanks to everyone who offered to help with testing on
older Sun-3X PROM revisions.  Here's what you do:

Download this file:
  ftp://ftp.netbsd.org/pub/NetBSD/arch/sun3/mdec/netboot
and try booting it (link /tftpboot/IPADDESS.SUN3x to it).
If you have a PROM older than 3.0 it will stop like this:
(This is a 3/260 running "hacked" Sun-3X test code!:)

    Boot: ie(0,0,0) -das 
    Using IP Address 172.16.100.76 = AC10644C
    Booting from tftp server at 172.16.0.14 = AC10000E
    Downloaded 17408 bytes from tftp server.
    
    >> NetBSD netboot [$Revision: 1.7 $]
    Debug level 1 - enter c to continue...
    Abort at 0024011E
    >c
    Using IP Address 172.16.100.76 = AC10644C
    netif_getether: Uh oh, PROM Rev 2.6
     sif [0xfeff448] 0xfef943a 0xfef9500 0xfef9056 0x69653a20
    Please record those numbers, then do: c<enter>
    
    Abort at 0024011E
    >c
    OK, calling sif->sif_macaddr ...
    Protection Bus Error, PC fetch at 002436E8

I hope you will not see that Bus Error at the end, but
instead it will just work on your old PROM revision.

Please report your results.  If we discover that older
PROM revisions support sif->sif_macaddr then the PROM
revision test can be modified appropriately.

Thanks,
Gordon