Okay, Flash 11 is out. It's good.
I have the following in /etc/adobe/mms.cfg:
Right-clicking on videos on Youtube gives me: "software video rendering, hardware video decoding." If I use the "full screen" button on the bottom right, then the video does not run at full frame-rate. BUT, if I instead right-click on the video and choose "pop out" to open it in a pop-out window, and then use my window manager's maximize button to maximize it, THEN, the video runs at full frame-rate while filling the screen!
Vimeo videos run fine full-screen. As far as I can tell, they run at the same rate-rate as they do windowed, just with more obvious ghosting.
This is with version 2.75 of the nvidia drivers, on a pure 64-bit system.
EDIT: it has the side effect of very
frequently locking up the computer.