Well, Here I am on my Macbook 5,1 in Snow Leopard with a 64-bit kernel and 6 GB of RAM installed. This test has been really, really, really odd. I booted up, and used eatmem to eat 6 GB of RAM across 3 calls. This pegged out my system, got me a huge page file, and other than being doggishly slow, things were running just right. Then I killed that and freed up the RAM, which is actually the point where I usually saw the kernel panics happen with the 32-bit kernel. Then I ran Parallels and Aperture. Man, Aperture is sweet with that much RAM, it really doesn't skip a beat!
Anyway, things ran great for about 50 minutes. Then, out of nowhere, Skype crashed on me. It wouldn't relaunch, either, just kept crashing during launch. Oddly, the crash reports showed some random unhandled exception rather than what I'd have expected--segfault. I tried to open up other random apps (iCal was first)... Those all crashed on startup, and now they were all segfaulting on startup. I quit all open apps (other than Finder), and tried to relaunch them. No dice, more segfaults. At this point, I was expecting a kernel panic any moment, but it sure never did. I gave up and rebooted, and that's where I am now, still with 6GB loaded.
I really don't think this is a firmware issue. It sounds like a kernel/kext issue, which would account for why switching kernels yields better results.
For what it's worth, this is with my initial stick of RAM from OWC/MacSales. They're sending me a box to return it in exchange for a replacement, which I will test with as well. I also need to check this stick out running solo (4GB single stick alone), assuming that I get it to break again in this session.