I "upgraded" from nvidia's 260.19.06 driver to 270.18 (which admittedly is still a beta). My ppsieve/cuda WUs used to run ~450-455 seconds on my GTX 570 (stock clocks across the board) and now they are in the 465-475 range, with no other mods. Not a big difference, but sad to see it slow even a little. I won't downgrade for now hoping the performance issues are resolved. My $0.02.
Ubuntu 10.10, up-to-date, with AMD 1090T.