I was able to get MPE (Premiere 6.0.3) running perfectly with my GTX 690 (310.90 driver) by going into the:
• NVIDIA control panel
• Manage 3D setting
• Find Premiere in the "program settings" tab
• Change "CUDA" to either of the GPUs (not both)
• Change "Multi-GPU Rendering" to single GPU
• Change "Multi-Display/Mixed GPU acceleration" to single disply performance
• Save changes and hope for the best!
Hope that helps!
-Keith