>>> ae0 at nubus0: address 00:00:00:00:00:00, type Ethernet A-Series, 16k mem.
>> Looks like ae driver's having trouble getting the MAC address...  You
>> might want to take a look at the on-board ROM.
>
> Take a look at it?  Well, it has a sticker on it saying "SSI NB256 V1.0
> 004010 02E61F", and it's removable (not soldered on).
>
> I assume you want me to use some software to print out the contents...

  No, the sticker is enough. Its real Mac address is 00:40:10:02:e6:1f
00:40:10 is the manufacturer code for Sonic Systems.

>>> It's a Sonic Systems Ether TwP Mac II in slot D.  Works fine in MacOS on
>>> my IIfx & IIci without any special drivers.
>> 
>> Ok, so we know the hardware isn't broken.  Is it a NIC (DP8390) or a
>> SONIC (DP83932, etc)?
>
> I couldn't find any mention of *NIC, DP* or 839* on the card.  The data
> sheet (linked off of http://www.sonicsys.com/cards.html) says that the
> controller is a "Sonic Custom ASIC" (or "SONIC-T" for the CommSlot cards,
> which I don't have).  It also says that the ROM size is 256 bytes.  The
> part number (of the card) is 01-SSC-0403, claimed to also work on
> Centris', Quadras, and NuBus PowerMacs.

  The company website says that all cards have 16k or 64k of
onboard memory. This means they are 8390 based, and would be
supported by the ae driver. The SONIC chip has no external
buffer memory (it has some buffering inside the ASIC) and no
way to attach it.
  The CommSlot card from Sonic Systems has a SONIC-T, which
is a version of the SONIC chip with an integrated Twisted Pair
transceiver.

> I suppose the easiest solution is to buy another card anyway, since I'm
> going to have to put this back in the IIfx.  What's the cheapest 10bT card
> I can get? :)

  The cheapest would be to buy a used card. http://www.ebay.com/aw is
an online auction house, which has a Macintosh Hardware category.
Ethernet cards regularly are auctioned off there for $10-20. Of course,
since you are buying from someone who you will never meet, you
are taking a risk (you would have to pursue a claim across the country
instead of going back to a store and demanding a refund).