Another update! I purchased/installed a GTX 580 and re-ran the “cuda-supported” Xilisoft Converter Ultimate. Not surprised to see that ALL the system instability and crashes associated with the GTX 480 are gone! I will state again, this looks like the GTX 480 architecture is the problem. Given this discovery, I am not surprised that nVidia has completely ignored my inquiries.
I agree, that nVidia might be more responsive to requests like this.
I also had serious stability problems with my setup and nVidia’s own SDK code. See this thread, if interested. My symptoms have been similar to yours: random system reboots when a CUDA app was running, while good overall system stability otherwise, even under heavy load. After some experimentation, I realized that it was my power supply, that was causing issues. This was a brand new power supply of sufficient wattage from one of the most respected manufacturers, and still it was malfunctioning. All issues went away when I replaced my power supply with a different unit of the same kind from the same manufacturer.
The conclusion I drew from my experience was that symptoms like the one you describe can be related to a malfunctioning particular piece of hardware, such as power supply, motherboard or video card, rather than with the architecture as a whole. Even the best manufacturers’ QA fails once in a while.