Maltaannon notes an undated article by Studio 1 Productions, How to Unlock Adobe Premiere CS5 use almost any NVIDIA graphics card with CUDA acceleration.
Caveats on this unsupported modification and its origin on forums were discussed here in May in Preference mod widens CS5 CUDA acceleration. The new article above details additional tests with a wider range of Nvidia graphics cards and notes that power requirements may change with new cards. Having inexpensive hardware acceleration for a few layers with effects on DSLR footage has made some users pretty happy; conversation on the mod continues at the Adobe forums.
Update: there are some instructions for the same on the Mac at Insanely Mac in How To Cuda / Mercury Engine on Premiere CS5 / Snow Leopard [ lower end graphic cards ].