Suppose my opponent has a Darksteel Colossus in play, and I have Attrition and no creatures. During my turn I cast Act of Treason to gain control of the Colossus and attack with it. So far I haven't done anything controversial.
Now, at this point what I want to do is sacrifice the Colossus to my Attrition, so that my opponent doesn't get him back at the end of the turn. However, Attrition's ability requires a target, and the Colossus is the only potential target on the battlefield. Is it legal to activate the ability and target the Colossus, even though I also want to sacrifice him as part of the ability's cost?
In other words, can I target a creature with an ability, and also sacrifice that same creature as part of the ability's activation cost?
(The ability won't resolve, of course, but that was never the real goal. Normally it would be better to target another of the opponent's creatures, but in this case he doesn't have any, so the Colossus is the only potential target.)