This is a hardware limitation on all cards up through the GeForce 500 series, not a software bug. Even when no 3D programs are running, the driver will operate the GPU at a high performance level in order to efficiently drive multiple displays. In the case of SLI PCs, the second GPU will always operate with full clock speeds, in order to efficiently drive multiple displays. This is a limitation that is common to GPUs from all companies.
GeForce 600-series and later cards have implemented architecture improvements that allow them to efficiently drive multiple displays without going into high-performance mode.