Venture Beat [Web Link initially reported the incident] in which it appears that Google, [Web Link Blippy] and a Midwest bank were all partly to blame.
Google reportedly said that its Web crawler should have automatically deleted the card numbers from its cache much sooner in a second crawl of the site which would have found the numbers deleted. But Google also points out that it provides webmasters with tools to correct such problems and that the numbers would have been deleted from Google's cache sooner had those tools been used.
Blippy was [Web Link profiled by the New York Times] just hours before the news hit the blogosphere. On its [Web Link Website] the online start up says it aims to be a "fun and easy to way to discuss what everyone's buying" and appears to compete with sites such as Amazon.com where people rate and review products.
CNET has a summary of the story [Web Link here].