My solution is more of a workaround, and it does not involve setting the NR_OPENGL_RENDER=1 environment variable. As mentioned earlier, this variable fixes corruption but causes sluggish render refreshes in the viewer if you zoom in on your work or use any procedural effect like a gradient or checkerboard pattern. But no more of that!
Run Shake 2.5 in a virtual machine!
I found that if you run Shake 2.5 in VMWare or Microsoft's Virtual PC, the VM's crappy graphics drivers are "old" enough to prevent the corruption. I tested a checkerboard pattern node, applied a blur filter, and got real-time feedback. Works like a charm.
Ironically, my only fear is that one day the graphics drivers in virtual machines will improve to a point where this no longer works, once again causing corruption unless you set NR_OPENGL_RENDER=1.
Running Shake in a VM can be taxing on your system resources and would probably be slow for rendering. So you could do your work in the VM, and then render outside in the host operating system, using all CPUs.
BTW, VirtualBox and VirtualPC are free VMs. VMWare Player is also free, but requires outside tools to make a virtual system.
If someone cares enough to try, maybe there is a low-level video driver tweak that can force Shake to run without corruption in regular Windows XP/Vista. I spent a little time going through both nVidia and ATI tweak tools but couldn't find anything that worked.
Also, some people may be able to dial down the Hardware Acceleration slider in the Display -> Advanced -> Troubleshoot tab or by running dxdiag from the command line. In Vista, nVidia/ATI drivers lock me out of being able to control this setting, so I can't test it. Pooh.