http://gristmill.grist.org/story/2007/3/12/63111/0928#31

The HVDC grid is very important.  I think that high voltage capacitors, either built into the power lines or in separate facilities along the grid, would provide enough storage.

And then there is distributed generation from biogas digestion used in solid oxide fuel cell/turbines for backup power.  The methane release prevented, waste water recycled, and organic fertilizer produced with systems like this are all great byproduct benefits.

And the fuel cells also run on natural gas. The ultimate fossil fuel backup energy supply.  There is enough of this source for many decades converted underground from coal and oil.

And I think the cost of wind power will drop signifigantly with mass production and a switch to wind machines three times the size of the current largest multi-megawatt machines.

Mounted on the nearly deserted northern great plains and offshore on floating wind/wave power platforms. 50,000 of these larger scale machines could take over 25% of baseload power generation.

Conservation and distributed solar, small to medium wind, and biogas to fuel cell generation could take care of the rest, even with a massive shift to plugin vehicles.