According to Wikipedia, a Billion means the same both in UK and in the US, and it's a "thousand million", or 1,000,000,000. Both use the so-called "short scale". However, some countries use the "long scale" in which a billion is indeed 1,000,000,000,000, as twiceAweek pointed out, however neither the US nor the UK use this system anymore.
http://en.wikipedia.org/wiki/Billion_(word)
The long and short scales are two different numerical systems used throughout the world:
Short scale is the English translation of the French term échelle courte.[1] It refers to a system of numeric names in which every new term greater than million is 1,000 times the previous term: "billion" means "a thousand millions" (109), "trillion" means "a thousand billions" (1012), and so on.
Long scale is the English translation of the French term échelle longue. It refers to a system of numeric names in which every new term greater than million is 1,000,000 times the previous term: "billion" means "a million millions" (1012), "trillion" means "a million billions" (1018), and so on.