See this Walmart warning: http://www.walmartstores.com/PrivacySecurity/10840.aspx?p=9620 and then follow this link (found in the Walmart article) to information on how to forward the text message to your cellular carrier so they can investigate it: http://www.walmartstores.com/PrivacySecurity/10852.aspx?p=9534
Here's more information from Walmart on this scam: http://www.walmartstores.com/PrivacySecurity/9309.aspx