My 24" iMac (aluminum, late 2008) is on a desk in front of a big window looking out over a lake. If I use it before 2pm I must lower the blinds on the window of the sun will hit the black plastic back of the computer and it will very quickly spike the temperature. Never actually measured it, but the fans blow like a jet engine and the entire case gets hot to the touch. Not good.
I would likewise recommend keeping your MacBook out of direct sunlight whenever possible.