It appears in the most recent release on the approvals for Medicaid that you have to have a BMI of 35 , and also have a medical condition.
"The Centers for Medicare & Medicaid Services (CMS) has determined that the evidence is adequate to conclude that open and laparoscopic Roux-en-Y gastric bypass (RYGBP), laparoscopic adjustable gastric banding (LAGB), and open and laparoscopic biliopancreatic diversion with duodenal switch (BPD/DS), are reasonable and necessary for Medicare beneficiaries who have a body-mass index (BMI) > 35, have at least one co-morbidity related to obesity, and have been previously unsuccessful with medical treatment for obesity."
Here's a link:
http://www.cms.hhs.gov/mcd/viewdecisionmemo.asp?id=160 
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