'Cornerstone' shares will offer the chance for large investors, not normally general members of the public, the chance to buy into large chunks of GM. The company and the US Treasury is worried that overseas investors may disrupt what little stability the company has left. This could certainly be true in the evident of a political dispute between America and the other country that buys in.
According to the Wall Street Journal, government officials are suggesting the company 'filters' which large cornerstone investors can buy the shares. Reports also say the company might need to limit the number of foreign investors invited to take part in the IPO.
For further reassurance in the company, the new GM CEO, Dan Akerson, has told the board that he'll be staying with the company for the long term. Unlike the previous CEO who just lasted 18 months, Akerson was said to present a number of incentives at a recent meeting which would take effect over the next few years. This was a way that Akerson could prove to investors that he would be with the company to live out these incentives and provide long-term commitment in the company's full recovery.