The odyssey is over. I installed the newest CUDA 5.0 development driver (which is a leaked one not available to the general public), version number is 302.59 and it's just a FEW days older than the latest WHQL. That CUDA 5.0 driver is available for Vista/7 x86/x64 as well as WinXP x86/x64. On Vista/7 it brings users full DirectX 11.1 support, and on Windows XP and Windows XP x64 Edition it finally brings... SLI!
The only thing not included in the devel driver is PhysX, so users can either download the full PhysX SDK to get access to that, or just install the PhysX package of a regular WHQL or Beta driver.
The end result after months of fear and waiting:
Edit: Addendum: It's not fully functional. Currently, OpenGL applications work flawlessly, but Direct3D applications produce a red screen with the lower tenth of the screen flickering between red and black (not the known "Red Screen Of Death", as the machine does NOT lock up). However, Direct3D also works when ran in windowed mode. A weird bug.
Edit 2: Addendum: Might be RSoD after all. After reading how RSoD behaves exactly, it seems the symptoms are 100% the same. So RSoD also won't lock up the machine, you can just Ctrl+Alt+Del out of it and kill the offending application. Exactly what I'm seeing. Only that it happens exclusively in SLI for me.