Okay, I have been experiencing this problem too.
I'm running Windows 7 Beta 1 64bit on my HP Pavilion laptop, which has an NVidia Geforce 8400M GS in it.
Running DirectX based 3D applications, or games in D3D mode instead of OpenGL mode, actually gives superior performance than under Vista, but comparable to Windows XP.
Running OpenGL apps in windowed mode is fine, but there's an obvious framerate drop.
Going fullscreen using OpenGL causes huge amounts of flickering, but the framerate goes like it should, and, the flickering doesn't show up on screenshots and videos, which render correctly.
This is confirmed to occur using the v169 drivers available exclusively for Windows 7 through Windows Update, AND using the "NVidia Notebook Drivers" for Vista 64bit (v179) available from the NVidia website.
I'm really curious as to what's causing this, and if there's a fix. :/