For technical reasons you can only use one adapter as supporting multiple adapters would come at a considerable memory and cpu cost. This is especially true for videos where you would end up having to decode the video using cpu only which even on a dual core machine would use very high levels of cpu (probably over a single cores worth)
Neil, without breaking my NDAs, I can say that right now, I'm using multiple GPUs to play and accelerate multiple simultaneous hi-def video streams on dual and quad core machines under Vista. As long as each stream is piped to a different adapter, and you have one codec PER stream (which is a player thread issue), this approach scales linearly by GPU and CPU. The only thing I can't do is make them desktop wallpaper dreams cause this project doesn't need that coded and I'm not really looking to get into the software business.
But, because these data streams are already MPG2 compressed (and assuming you cache the whole video in RAM, which makes the most sense in these days of 4 gb systems), one could do something similar to the DLP software players like...
http://www.3dtv.at/Index_en.aspx
This software can read two video streams, interleave them in real-time (for the DLP matrixing) and play the result across 2 monitors per adapter, etc. I'm sure your comments are 100% accurate for Windows XP, but I do believe Vista has addressed these issues at the core level.
Regardless, as a layman's example, I just played 3 1080p streams on my 3 monitor/2 adapter system. To get around the windows media player restriction of having one instance open at a time, I played one 1080p source with VLC on monitor 1 (adapter 1 primary), another on windows media player classic on monitor 2 (adapter 1 secondary), and a third playing in Vista's latest Windows media player on monitor 3 (adapter 2 secondary). This is a quad core 6600 and each video is playing off a different drive. I can't cache them to ram because I chose full length movie sources for the test. All three played at full speed, sound mixed for all three, etc. CPU hit was heavy, but these are all MP4s at 1080p, and using three different players makes this HARDLY the ideal configuration, hahaha.
As a second layman's example, I just played two instances of VLC in DirectX "Desktop Wallpaper" mode, one on monitor 1 (adapter 1, primary) and the other on monitor 3 (adapter 2, primary) and again both were playing CPU intensive mp4's at 1080p in realtime with sound streams. Again, the CPU hit was higher due to MP4s at 1080p, but there were no dropped frames, etc. MPG2 files have shown great GPU acceleration offloading compared to mp4.
And finally, you are probably already aware that DX11 will support multithreading within the GPU, so that would address this on a single GPU as well. So all of this should be even easier under Windows 7/DX11 next year.
I hope some of this info helps, even if it still means we can't have this feature any time soon. Thanks for all your excellent work so far on all the cool toys we all love so much. 
