Next to a Chinese restaurant in Burlington, Vt., lurks a quiet guardian of Wall Street — an obscure insurance company that is supposed to protect big-money investors in the event of a catastrophic failure of a major brokerage firm. A failure, for instance, like the one that brought down Lehman Brothers nearly 11 months ago. Now, after years in the shadows, the insurer, the Customer Asset Protection Company, could finally be put to the test, and questions are starting to swirl. The worry is that the company, which has never paid out a claim, might be unable to cope with...