Actually it IS in the platform release notes.
Windows release notes, halfway down: "Individual kernels are limited to a 2-second runtime by Windows Vista. [should that be all Windows versions, or just Vista?] Kernels that run for longer than 2 seconds will trigger the Timeout Detection and Recovery (TDR) mechanism. For more information, see http://www.microsoft.com/whdc/device/display/wddm_timeout.mspx."
Linux release notes: “Individual GPU program launches are limited to a run time of less than 5 seconds on a GPU with a display attached. Exceeding this time limit usually causes a launch failure reported through the CUDA driver or the CUDA runtime. GPUs without a display attached are not subject to the 5 second runtime restriction. For this reason it is recommended that CUDA be run on a GPU that is NOT attached to a display and does not have the Windows desktop [???] extended onto it. In this case, the system must contain at least one NVIDIA GPU that serves as the primary graphics adapter.”
But it really should be in the programming guide. Or at least in the reference manual in the section about the error code cudaErrorLaunchTimeout. Release notes are the last place anyone looks.