The 3/9/2012 Houston Chronicle reports, "More Texas doctors opting out of Medicare". Only a few states track this particular statistic, but I suspect this reflects a nationwide trend.
As for why conscientious physicians might wish to opt out, Dr. Kathleen Brown explains in her recent essay, "Exiting the Game".
Note: The federal government makes it difficult for patients and doctors to opt out of Medicare. If a patient wants to opt out of Medicare and seek their own private arrangements with willing physicians, they may lose their Social Security benefits -- something many seniors can't easily afford.
Conversely, if a doctor wishes to completely opt out of Medicare, then they basically have to give up all Medicare reimbursements for 2 years. So effectively, they have to be willing to sever relations with all their Medicare patients, even if they just want to work out a non-Medicare arrangement with one patient for one particular operation or treatment.
(There is a way in which doctors can sometimes take Medicare and other times not, but to do so they have to agree to accept less than the Medicare rates, which is already pretty low. Interested readers can find details at, "Medicare Participation Options for Physicians".)
More broadly, the government has a deliberate policy of making it difficult for patients and doctors to engage in private contracts outside of Medicare. Attorney Kent Brown covers this in, "The Freedom to Spend Your Own Money on Medical Care: A Common Casualty of Universal Coverage".