Guys, I'm hopeless. The last couple of times I've gone out to eat with the wife, I've ordered a nice cold Guinness on tap. I haven't found anything that compares. BTW, I just found out the idea that Guinness is to be consumed warm is a myth. In Ireland apparently they prefer to drink their Guinness cold and Guinness even recommends drinking it cold. I just love learning new important facts of life!
As the resident brewer and beerologist, I feel inclined to speak.
Quite a bit of clarification is needed regarding this subject, and temperature and defined temperature are crucial to understanding the differences. Actual Celsius or Fahrenheit degrees are needed when trying to tell someone if you should drink it warm or cold. How cold is cold? How warm is warm?
The old adage, that the British drink their beer cellar temp (often mis-diagnosed as warm) and Americans only drink their beer ice cold, is often misunderstood. Cellar temps in Europe are actually very cold. Just ask Buggy when he goes down there wearing only his knickers. Not all Americans drink ice cold beer, nor are most American beers served ice cold. Definitions of warm and cold need to be defined only in terms of temperature. You can decide whether that temp is considered warm or cold for you.
In saying that, different beers are served at different temperatures.
1. Lagers and Pilseners are served at the coldest temperatures, around 41F, 5C
2. Most craft beers (IPA's, Amber ales, hefeweizens, Porters and Belgians...) are served around 46F, 8C.
3. British real ales, bitters, barleywines, Abbey beers, Old Ales, and Baltic Porters should be served around 53F, 11-12C
CAMRA (Campaign for Real Ale) describes an appropriate temperature for Guinness to be served between 50-53F, 10-12C. Whether you consider that cold or warm is a personal preference.
The biggest thing to remember is that the colder the beer is served, the less flavorful it is. Hence, the warmer the beer is, the more flavors you will taste. That's why very complex beers like English Old Ales and Barleywines are served not ice cold so you can taste the immensity of flavors. Often, you'll see Budweiser advertised as Ice Cold Beer, that's because you need to drink it Ice Cold so you don't taste the Preservatives and other non tasty components of that beer. And remember, there is no such thing as room temp beer... Room temp is 65-75. England never reaches those temperatures even in the summertime.

(Burp) - Now, back to your beer drinking everyone.
-BundesBiers