The list of potential grounds on which a sentence may be vacated is large.  The selective list of grounds given here is grouped in certain categories, for ease of consideration. 

POST CON RELIEF " FEDERAL " MOTION FOR REDUCTION OF SENTENCE
Freeman v. United States, ___ U.S. ___, 131 S.Ct. 2685 (Jun. 23, 2011)(defendants who enter into FRCP 11(c)(1)(C) agreements that specify a particular sentence as a condition of the guilty plea may be eligible for relief under 18 U.S.C. 3582(c)(2), allowing review of sentence when sentencing guidelines have been amended downwards, and which applies retroactively).

Ninth Circuit

POST CON RELIEF " FEDERAL " SENTENCE " MOTION TO REDUCE SENTENCE FOR SUBSTANTIAL ASSISTANCE " RULE 35
United States v. Tadio, ___ F.3d ___, 2011 WL 5839660 (9th Cir. Nov. 21, 2011) (affirming reduced sentence under F.R. Crim. P. rule 35(b); court properly considered factors other than assistance, including those listed in 18 U.S.C. 3553(a), once court determined that the defendant provided substantial assistance to the government).