After a few attempts at trying to manipulate the vendor and device ids to make the Apple drivers work, I decided to write my own from scratch.
Well, more precisely, I decided to blatantly rip off of the GPL'ed forcedeth driver for Linux, which someone had mentioned earlier (but I had a really difficult time getting a copy of the source code on Google, I only managed to get several old versions so far). But I figured 2000 odd lines of code isn't too unreasonable and I could figure it out mostly, and also figure out enough of OS X's innards so I can port it.
Luckily, I was right, and it's partially thanks to the Apple team for providing a surprisingly friendly platform to code... device drivers on. I've never coded one in my life, and I've never coded for Macs, either, but I was able to get the hang of it pretty quickly. (However, I must say the documentation wasn't the best... I mean at least do a better job of hyperlinking it! And also I had quite a fun time trying to get the provided interrupt request handler to work right... it actually crashed my computer a lot more than the "unsafe" method I used earlier did)
The driver is pretty crappy, I have severe doubts about its stability, and indeed, I've had to restart my computer a lot during the development process, but having your code running at the kernel level doesn't give you a lot of room for error. But, hey it works (sort of... I think it might still want to murder USB if I try to unload the kext... "unsafe" method didn't have this problem). Also, I wasn't able to implement the latest and greatest features. Like scatter-gather, TOS and so forth. For those things, I'd probably have to actually talk to a real person to figure out what's going on instead of just looking at the code (which is pretty much not commented at all).
Anyway, I'm not how much interest is there (all the other nForce4 people seem to have other LAN cards or have the Deluxe or Platinum version and using the Marvell Yukon driver), and the driver so far is still pretty humiliatingly unstable (though I haven't tested it out extensively). If you want the source, it'll be all GPL'ed and such.
But I mostly wanted to brag about how I wrote a OS X driver. It doesn't seem like that happens a lot... Pretty good for a Apple newb, eh?
The driver attached should now be fairly stable, but as with all OS X drivers, there is no guarantee of support. Complete system freezes have been known to occur with older versions and may reoccur, so avoid doing critical work on OS X. Source code is included for the sake of future generations. Here are my recommended instructions for getting it working on your computer:
1. Download the attached driver
2. Extract the contents onto the desktop
3. Open the Terminal application in the Applications/Utilities folder
4. Type "tail -f /var/log/system.log" to obtain debugging output for the first run.
5. Use the menu bar to open up a new Terminal window.
6. In this new window, type "cd ~/Desktop/forcedeth/build/Release"
7. sudo chown -R root:wheel forcedeth.kext
8. sudo chmod -R 755 forcedeth.kext
9. sudo kextload -v forcedeth.kext
10. Observe in the log window what happens. If there are no errors, and you eventually see the link going up with a proper speed, then the driver will work with your hardware. Otherwise, paste the log in this thread for help.
11. Test out the driver by browsing some websites, etc.
12. If there were any problems in steps 10 or 11, type "sudo kextunload forcedeth.kext", then repeat steps 7-11 for forcedeth-nockd.kext
13. To install the driver, type "sudo cp -R forcedeth.kext /System/Library/Extensions"
14. Lastly, update the extensions cache with "sudo kextcache -k /System/Library/Extensions"
EDIT: I needed to do some work on OS X again, so I installed Leopard. The original version of my driver seemed to not work/immediately kernel panic so I made a small change. I think it's probably the same change I made awhile ago when I was trying to fix it for stability. I also cleaned up the plists a bit. But if it didn't work for you at all before, it will probably not work for you now either. But if it worked for you before, this version should work better.
It was compiled under 10.5, so it might need a recompile for your version of the OS.
EDIT2: Replaced old version with a newer version that fixes a bug that caused kernel panics for me before. After some work figuring out how to get panic logs, I traced it to the occasional failure of replaceOrCopyPacket. I did a quick Google search to see why that could possibly happen -- only it lead me right back to this thread, where someone named chuttenh had already traced it to the very same problem. D'oh! I can't keep track of this huge thread. Anyway, I posted my version of the fix. There's some weirdness with refilling the ring with "available packets" after errors that I think may come up with earlier versions and chuttenh's version, but I am not sure. His version has a few other changes, I think, and is available from his post somewhere in here.
EDIT3 I merged MeDevil's changes for MCP61 and called it 0.3 (thanks, MeDevil!). I cannot test these changes myself, but according to MeDevil, it should take care of the waking up from sleep problem and backwards MAC address issue for MCP61. Note that the device ID is hardcoded in there, so if anyone has these issues and do not have MCP61, change where it checks specifically for 0x03EF, stick in your device ID, recompile and see if it works for you. If it does, tell me and we can add it. (EDIT3.1: Updated 0.3 to actually add MCP61 to the Info.plist, forgot to do that last time. Oops!)
EDIT3.2: Github repository up at http://github.com/pl.../forcedeth-osx/. Issue pull requests whenever. It also has a wiki if people would like to share their experiences with the driver there.