I plan to do the same, but can't find the correct hardware too.
This is the only Ethernet card in a PCI-e Mini Card format that I've seen:
http://www.bvm-store.com/ProductDetail.asp?fdProductId=547
They have a dual-port card too:
http://www.bvm-store.com/ProductDetail.asp?fdProductId=548)
The latter would be great for LACP (link aggregation). I am not sure that you can aggregate the on-board interface with another one in an add-on card, but I could be wrong.
The problems that are keeping me from testing the thing:
1. The card is a little overpriced (£39);
2. I would have to pay shipping from UK to Brazil (where I live), which would cost almost as much as the card;
3. There's a big chance that it won't work. 1x PCI-e Mini Card slots can be implemented with USB signaling, and the card would not work if it's not designed for this kind of slot;
4. Maybe Apple limits the hardware into accepting only supported Airport cards in the slot (there are documented cases of this kind of limitation with other manufacturers);
4. BVM has a strange warning on their dual port card web page: it will work only with some Commel motherboards. I don't know if it applies to the single port card too and have not called or emailed them to check.
Why I should give it a try:
1. I don't believe Apple would use a half-baked PCI-e Mini Card slot in their computers;
2. The BVM card uses an Intel chipset, for which -- hopefully -- there's a driver for Mac OS X. I tested a Compaq dual-port 10/100 Ethernet PCI card in a G3 with OS X 10.4.11 and it worked right away -- the drivers already come with the OS. Also, BVM say the card is a Mini-PCI Express (a.k.a. PCI-e Mini Card);
3. It's the only solution I've seen so far.
If you're willing to put the Mini in another case, you can get a PCI-e to PCI-e Mini-Card adapter and use a regular 1x PCI-e card (the ones for desktop computers). This is one example:
http://www.hwtools.net/Adapter/PM2C.html
I was thinking about modding my Mini in a 1U case (7" deep) and go this route. This way, I could use a regular PCI-e card and also a 3.5" hard disk. I wouldn't be proud to show the internals to anyone, though. :lol: