According to a report from Automotive News, a number of factors can cause the ignition to switch out of the run position, including weights on the key ring, rough or bumpy roads or other "jarring" events. Any of these situations could lead to some vehicle components not functioning properly.
There have been five fatal front-impact crashes that took the lives of six people, although as a GM spokesman noted, all five of the crashes happened off road and at high speed. In each of these cases, though, the lack of airbags wasn't the only lethal factor - alcohol and failure to wear a seat belt also played a role. Outside of the fatal accidents, there have been 17 other crashes where airbags didn't deploy. It's unclear if any of these crashes were caused by the engine shutting off.
According to AN, GM is contacting dealers to report to their local dealership for a new ignition switch. Naturally, all work will be done free of charge (presumably, Pontiac owners will need to report to Chevy dealers). Customers that haven't or aren't able to report for service, meanwhile, are being asked to remove "non-essential items" from their key rings. 619,122 of the affected cars are registered in the US, while 153,310 are up north, in Canada. South of the border, 6,130 cars are affected.
Once the National Highway Traffic Safety Administration has posted its recall bulletin, we'll be sure to update this post.