United States v. Diaz-Tejada, No. 07-4594-cr (2d Cir. Feb. 25, 2009) (found here)
I haven't seen much about attempted cooperation as a basis for a non-Guidelines sentence since the Second Circuit recognized that ground in United States v. Fernandez, 443 F.3d 19, 30 (2006) (described here). But, in Diaz-Tejada, the Second Circuit came right out and said it: "Diaz-Tejada is correct that attempted cooperation is a factor to be considered under [Section] 3553(a)."
In his case, though, the Second Circuit found that the district court had, in fact, properly considered his attempted cooperation in determining and imposing sentence.