More and more cracks in government gun prosecutions are starting to develop as creative defense attorneys seek to apply the Supreme Court's Second Amendment holding in Heller in federal firearm prosecutions, such as this U.S. v. Engstrum district court opinion described in The Volokh Conspiracy as follows:
A federal court holds that someone being prosecuted for possessing a gun after having been convicted of a domestic violence misdemeanor is constitutionally entitled to present an affirmative defense "that he posed no prospective risk of violence" (which I take it must mean no prospective risk of violence beyond that posed by the average person). The jury would thus be instructed that, if it agrees with the defendant that he posed no prospective risk of violence, it should acquit despite the flat prohibition imposed by the statute.
See also this Volokh post on pro-defense, district court Second Amendment dicta and Prof. Berman's Readings for Second Amendment fans. More Second Amendment Sources
Closer to home, this EDCA Motion to dismiss 922(g) charge (though unsuccessful before LKK) provides a sample for others who want to raise Second Amendment motions in 922(g) or other firearm prosecutions.