Here is some information from the Consumer Guide to Bariatric Surgery (this info is two years old but I'm pretty sure it's still accurate).
"Many private insurers require a letter of medical necessity from a doctor before they agree to pay for weight loss surgery. Medicare, however, does not require such pre-certification or pre-authorization. A surgeon will submit the claim to Medicare after he or she has performed the surgery on a candidate who has met all the Medicare requirements for weight loss surgery. Some surgeons may ask Medicare patients to sign a contract stating that they will pay for any costs that Medicare does not cover after it processes the claim."
www.yourbariatricsurgeryguide.com/insurance/
Now, this is if you have traditional mediare, Parts A and B. If you have an Advantage Plan, Part C, you may need pre approval.