Today the Eighth Circuit held that the Feeney Amendment's requirement that the government approve of § 3E1.1(b) could not be applied retroactively: the requirement violated the Ex Post Facto Clause. (Details here). The requirement also violates separation of powers.
The doctrine of separation of powers imposes structural limitations on each branch's exercise of power, is implicit in our constitutional design, and dates back to Montesquieu. The Founders were intent on creating a Constitution that limited each branch's power because the concentration of the legislative, executive, and judicial power would lead to tyranny. It is the nature of government to accumulate power for itself. By dividing this power, in the words of James Madison, ambition would "be made to counteract ambition." Certain provisions of the Feeney Amendment allow the executive branch to encroach on the judicial power and should be struck down.
Another issue arising under separation of powers asks whether vesting significant sentencing decisions in the hands of the executive impermissibly abrogates the judicial power. Under the American constitutional system, there is to be a strict separation of powers between each co-equal branch of government. But in practice, "[w]hile people sometimes refer to the three branches of the federal government as a three-lawyer cake, it is more accurate to think of it as a marble cake." Thus, a prosecutor's decision on what charges, if any, to bring will ultimately impact the available options at sentencing. And the prosecution holds almost absolute discretion in this area.
However, the Feeney Amendment conditions the trial court's sentencing decision upon prosecutorial approval. For example, under §5K3.1 the judge may issue an early disposition reduction only "[u]pon motion of the Government" and when "authorized by the Attorney General of the United States and the United States Attorney for the district in which the court resides." Under §3E1.1(b)(1), the trial court may give the defendant an additional base level increase of one only "upon motion of the government [ ]." This conduct seems to confer upon the executive power beyond its proper prerogative and almost amounts to an "executive veto" of judicial sentencing decisions. And like the "legislative veto" Congress improperly reserved for itself in INS v. Chadha, the executive's effort to dictate sentencing decisions should be struck down as unconstitutional.
 John E.
Nowak & Ronald D. Rotunda, Constitutional Law §3.5 (6th ed. 2000).
 Bordenkircher v. Hayes, 434 U.S. 357 (1978) (finding no Due Process violation when a state prosecutor reindicts on more serious offense when the defendant did not plea guilty to the crime with which he was originally charged.)
 U.S.S.G. §5K3.1.