FarmdocDaily just released today an analysis of the base acre reallocation options under the new farm bill. The report gives a very good analysis of the base acres by the various major crops and the variances between base acres and planted acres for 2012. Here are charts of the major crops by base acres and planted acres of 2012 (taken directly from the report):
As you can see, wheat base acres are substantially higher than planted acres. Many of the base acres located in North and South Dakota and other "fringe" areas of the corn belt have now been planted to corn and beans over the last several years. In the percentage terms, the difference for barley is even more dramatic. Barley's planted acres is 3.6 million, whereas base acres are 8.6 million. The difference is 5 million acres or about 58% based on base acres or about 139% based on planted acres.
There is a good table showing how the average acre planting history for 2009-2012 will be used to determine the reallocation of base acres.