Are you able to open a single wow session, put it in a window, turn on FPS. Should be like 50 or what ever you usually have it on - and move it aroudn to each and every monitor?

In Vista when a single pixel moves into a monitor from the secondary GPU the frame rate drops to like 10 or so. Move it back to the primary GPU and frame rate recovers.

If you can move it around with no problems then it will tell us for sure if Windows 7 fixes the whole secondary GPU not-rendering thing.