Green Power How-To

From Treehugger, an overview of a number of ways to green your power purchases. I still struggle with the concept of green tags: I understand how they work, and encourage people to invest in green power this way, but it still feels like a means of tapping into "liberal guilt..." Also, at what points are these certificates no longer tradeable. Doesn't the "green effect" that one buys expire at some point? Or, am I confusing my green tradeables here...?