The results:
In 3Dmark03, MCP78S 2683, 8400GS scored 5800.Enabling Hybrid SLI caused a performance drop of more than 100 3DMarks. It looks like NVIDIA still has some driver tweaking to do before these motherboards are available.
For hybrid SLI, it can operate in two modes, Energy Saver mode or performance mode.
In Hybrid SLI Energy Saving mode, it scored 2700.
In Hybrid SLI Performance mode, the score is 5700. This is shocking as it is slower than a discrete 8400GS graphics card performance.