General Motors Co. (NYSE:GM) has come a long way since its 2009 lows, using bankruptcy to trim costs, streamline operations and regain sales momentum. But skeptics remain worried that the company, despite its rejuvenated balance sheet, could still fall victim to the mistakes that plagued it for decades. And January sales figures announced last week will likely do little to quell those fears.

Detroit-based GM reported an impressive 21.8% year-over-year rise in sales, topping analyst targets and besting gains by rivals Ford Motor Co. (NYSE:F) and Toyota Motor Corp. (NYSE:TM). But those gains came at a price: GM increased spending on marketing promotions by nearly 16% from December, to $3,762 per vehicle sold, according to Edmunds.com, even as Ford was reducing its average incentive by 13%, to $2,859 per vehicle.

GM officials disputed suggestions that the incentives fueled their increases, with company U.S. sales chief Don Johnson on a conference call saying "we're not behaving any differently from anybody else," though admitting "we did catch our competitors a little flat-footed."

Johnson pledged GM will remain responsible, and not fall back into a cycle of price cuts that drained profitability from U.S. auto sales and helped land GM in the mess it was in. But in a note entitled "Is this how price wars start?" Barclays Capital analyst Brian A. Johnson sounds the alarm, saying GM's use of incentives may mark "a dangerous first salvo in a heretofore quiet pricing front."

The question is once the incentives start, where do they end? Toyota, the analyst notes, has pledged to offer incentives that "will be leading the market" to win back share lost to high-profile recalls in 2010. GM also has ample motivation to keep sales momentum strong, with lead shareholder the U.S. Treasury needing an average price of $53 -- 49% above its current $35.60 -- to break even on its bailout of the automaker.

Barclays' Johnson notes that a price war would be good news not just for consumers but auto suppliers as well, thanks to inflated volumes. But for manufacturers already facing increasing commodity costs and pricey launches of new products such as GM's Chevy Volt, the long-term consequences of a price war could be devastating to profitability.

GM, in marketing itself to Wall Street ahead of November's initial public offering, was quick to distance itself from its past. Executives this week pledged the automaker would "continue to be very judicious" with incentives. Investors had better hope so.