United States v. Guerrero-Velasquez, ___ F.3d ___ (9th Cir. Jan. 19, 2006) (an Alford plea, in which the defendant enters a guilty plea while maintaining his innocence, is nevertheless a guilty plea under Taylor v. United States, 495 U.S. 575 (1990)).
http://caselaw.lp.findlaw.com/data2/circs/9th/0530066p.pdf