The idea is to save power in PCs and laptops (most of which ship with GigE cards now) when LAN links are idle, or not utilizing full bandwidth. Researchers estimate that U.S. companies could collectively save $450 million a year in power costs by using such a technology.
Rather than doing a single link-detection and speed-detection upon an interface become active, the proposal would see devices that do real-time checks, allowing a link that is being used very little to throttle down to 10mbps, for instance when just casually surfing, but then increase up to 1Gbps when streaming a video from a media machine in the other room.
It's definitely a challenge, from the speed at which auto-negotiation currently completes to the various different standards hardware has. While the amount of power that Ethernet draws overall is just a drop in the bucket, it's a big enough drop to get others interested.