One proposal on the table is to disallow PV feed-ins on that day to prevent the grid from collapsing.The German power grid operators are dreading March 20, 2015. On this day Germany will see a partial solar eclipse during the morning. Should there be no clouds in the sky at this time, all solar power generating systems all over the country would be feeding in drastically less power into the grid in just a matter of minutes – and the grids would become dangerously destabilized.
Hopefully they don't do that and will instead pursue a technical solution.
In any case, it should be an interesting experiment which has never been carried out on such a scale. I expect we will learn something we did not previously know.