Re: Xorg, Nvidia -> black screen of death.
Only other things I can think of are:
Make sure you aren't using 4kb stacks in your 2.6.x kernel. (reports of AGP not working if you do have it enabled)
You can try using the driver w/o 3d support by commenting out the "glx" line in your xorg.conf file. (or XFree86.conf).
Not sure if this'll help, but you can try using APM instead of ACPI. It's not near as powerful, but I've not had near as many problems with it.
Overall, I still think this is just something we'll have to wait for the next driver release before it works.
Also, opterons are still bleeding edge, despite what anyone says. Look for bios updates for your motherboard, and any opteron/X86-64 specific kernel patches that haven't yet been applied to 2.6. Oh and you ARE using the x86-64 specific nvidia driver right? Not the generic ia32 one? Just checking.
Do not meddle in the affairs of kernel programmers for they are subtle and quick to anger.