Well its off topic, but I have to answer any queries about such a national phenomenon!
I cant say for sure there is a temperature, I guess it has to be just right, not too hot , not too cold. Too cold and it hurts your teeth when you're taking a big swig of it, too hot and it loses its head and tastes of rotten malt before its finished.
I have just read the Guinness website and they quote 6 degrees C for draught and 3.5 degrees C for Guinnesss extra cold draught.

Gerard