Recently, Microsoft published a document "Guidelines for Graphics in Windows 7" which warns about possible problems with combination of video cards from nVidia and AMD/ATI. Microsoft openly calls nVidia and AMD/ATI an unstable technology.
nVidia Hybrid SLI and ATI Hybrid CrossFireX technologies use a combination of built-in cores and video accelerators to enhance video card performance.
Attention, Windows 7 will not support dual video cards. Reason for that (as stated by Microsoft) is technology made by nVidia and ATI are not stable and will cause unstable performance, crashes and frustration to users. Microsoft is sure that this issue will keep manufacturers from continuing distribution of such systems. Also, Microsoft warns that every re-configuration of video settings will require reboot and no one likes doing that.
When ATI Hybrid CrossFireX was tested with Windows Vista it performed very well and showed speed acceleration. No instability or errors have been reported. So why does Windows 7 experiences problems?
Note, it took only few days before Microsoft officially removed the official document off their website. Possibly they're overlooking their decision?