Guinness is served with a gas mix like NeoTrout said. Something like a 70% Nitrogen 30% CO2 mix. The stout faucet has a disc with small holes in it so the beer comes out with that cascading head on it. Since the CO2 is a small % of the gas it isn't carbonated as much as other beers. Real ale is served with a beer engine. They don't pressurize the beer like most, or all, beer you see in the rest of the world. CAMRA, or the campaign for real ale, saved cask served beers in the UK. I love real ale.
Sorry for the rant. I just feel pasionate about good beer and think others like to hear about it. Even when they really don't. hehe