Apple is using a live countdown timer for is 1 Billion App Countdown promotion.  It's almost real-time, and is a step better than when they did the 1 Billion Song Download a few years ago.

At the rate its going, they'll reach a billion before the end of the week!

Download an app and you’ll automatically get the chance to win a $10,000 iTunes Gift Card, an iPod touch, a Time Capsule, and a MacBook Pro.

Also, there a link to enter the contest without buying an App.  You can enter 25 times a day.