The Supreme Court of the United States handed down Peugh v. United States today. The individual in question was sentenced according to the harsher set of sentencing guidelines that were in force at the time of sentencing rather than the milder set that were in force at the time he committed the offences.
The majority held that this amounted to a retroactive increase in punishment contrary to the ex post facto clause. That seems about right to me.
Thomas J. has an interesting dissent (for four judges) in which he dismisses the guidelines as not of binding force and effect. The ex post facto clause applies to law, not to administrative guidelines.
This is a distinction more of form than substance, I fear. The point of introducing the guidelines is that they have normative effect and are likely to influence the exercise of discretion. If that is the case, their technically non-legal status should not be treated as dispositive.