The simplification rule applies to the plants that were put into operation after December 31, 2003. Income from such plants no longer has to be declared in income tax returns.
It is intended that the tax offices will approach the taxpayers concerned in writing when processing the tax return.
The BMF letter is available here
Further information can be found in the FAQ Taxes of Oberfinanzdirektion Karlsruhe under the heading "Photovoltaic systems and combined heat and powerplants".
The press release of the Ministry of Finance Baden Württemberg can be found here.