As I am sure you know, the Federal Torts Claims Act preserves immunity for the Federal Government in cases in which the conduct can be considered a "discretionary function." But, the case law has made it clear that not all conduct that is discretionary deserves immunity. Only that conduct which is discretionary in order to pursue a public policy based on economic, political or social concerns is considered the type of discretionary function that should be protected from possible liability - all in the name of protecting separation of powers. This distinction is key because otherwise the government could claim immunity for any and all conduct that was not mandated by regulations of some sort.
Day on Torts is reporting, however, on a recent case which appears to eliminate the distinction thus recognizing immunity for a ministerial decision which, although discretionary, should not be considered to be the type of discretionary decision that should provide immunity for the state. The case is called Kohl v US and it is available here.