BOFH:
Guinness makes several different beers. The stuff in the US in bottles is completely different that the stuff in cans, and neither is the same as the stuff sold in Europe. The bottled beer is over 1.3% stronger than the can, and is not nitrogen-infused (the creamy head).
I wish I could remember more... I used to know the whole (very interesting) story.
Cheers,
-Dave