They've been making the announcement all summer, and it does help keep the plane cooler while it's at the gate with the window shades down. They also have been asking passengers to fully open their air vents. I don't know how much it really saves on fuel since the planes are usually connected to ground air conditioning while at the gate. But we all know the ground air conditioning units at IAH can only do so much this time of year; often the planes are quite warm by the time boarding begins even with the air flowing. So having the window shades closed blocks the sunlight, and makes the cooling more effective.