Michael, with regards to Photosynth, there's no immediate benefit yet, but the more people who have a browser that supports CSS3's 3D capabilities, the sooner the Photosynth team can consider rewriting the Photosynth viewer to not even need a browser plugin to do 3D graphics.
Once Firefox 4 is released, I believe that all five major browsers will support CSS3's 3D abilities, but there will be a difference in performance among them. From what I've been able to tell, IE9 uses the GPU more consistently.
The big difference between the performance of the Direct3D Photosynth viewer and the Silverlight 2, 3, and 4 Photosynth viewers is that Direct3D was using your GPU and the Silverlight viewer was only using your CPU.
Being that I am not an expert in 3D graphics, I can't say that CSS3's 3D abilities are equal to DirectX 8's, but I am interested to see what the Photosynth team is able to do with the new generation of web browsers.