You can picture the scene. You're going through the barriers at Leicester Square, you're in a bit of a rush to get to work, you slam down your Oystercard and instead of the red "Seek Assistance" flash that you always ignore, the forecourt erupts with dancing girls wearing flourescent waistcoats and little else. A Big Brother tannoyed announcement booms "You are our billionth customer". Cliff Richard sashays over with his trademark sideways dance, singing "Congratulations". Followed by Sheriff Ken with a prize newt, who warmly shakes you by the hand and offers you free travel for the rest of your life. Transport Commissioner Peter Hendy, stops MySpacing and hands you a glass of champagne. Bob Crowe promises never to organise a strike on your line on days that haven't got a y in them (looks like another strike is brewing for Easter Monday).
In reality, I imagine that the billionth commuter (and it could have been you), probably experienced a delay on the Northern Line and had his or her face shoved in the armpit of a fellow commuter.
Well done (I think) to getting a billion passengers this year - the first time in the 144 year history of the London Underground.