Brian Burke at Advanced NFL Stats.com is working on a system of predicting win probablities in real time as games progress. Here's his explanation of the current beta,
The probabilities are based on over 2000 games from the past 8 regular seasons. Last week, the model was fairly basic and simply considered score, time, and possession. This week modifications for field position and seconds are included. You can think of each probability as "if the team with the ball had a first down at their current field position, this would be their chance of winning."
The Colts win probability before the Rosenfels fumble was .03, 3%. 3 out a 100 times will a team win in that situation. Amazing.