Two posts ago I wrote about Saddam's hanging, the Indian reaction, and the death penalty, observing in passing that Kehar Singh was hanged despite being guilty of nothing more than conspiracy.
Apparently, in the US (which executes about as many people in a week as India does in a decade), conspiracy is sufficient cause for the death penalty. This article on the Zacarias Moussaoui case (dated March 2006, but I just saw it) goes into more details. The prosecution's charge against Moussaoui is not quite even conspiracy: it was basically that he knew what would happen on September 11, 2001 and didn't try to stop it. (Eventually, he was awarded life imprisonment.)