Guinness Draught (cans or bottles) = Cold
Guinness Stout/Extra Stout = barely chilled

I've had several arguements with people over this. The 'purists' will always say that it needs to be served warm, like in Ireland. Most don't know that there are actually several varieties of Guinness.

The above statements on 'cellared temperature' also hold true to wine. Whites are to be chilled, and reds are to be served at 'room temperature'. Well, when those rules were made, 'room' temperature was 10-12c, not the 20-22c that it is now.