The WashTimes reports on the GAO telling us something we already knew:

Foreign nationals illegally in the United States are using identification cards issued by the governments of Mexico and Guatemala to avoid apprehension and deportation, a government report said.

According to the Government Accountability Office, weaknesses in U.S. government policy regarding the issuance of the cards, also known as matricular consular cards, failed to prevent their delivery to and use by illegal aliens...

As pointed out Their Money or Your Safety, the consuls who distribute these cards freely admit they aren't concerned about the recipient's immigration status. That post also discusses how the Bush administration worked to allow banks to accept those cards.