Is the ACA a path to Single payer? What about a Medicare buy-in, or a Public Option?

I’ve just been part of a Twitter exchange as to whether or not the ACA is a pathway to a single payer plan? Many single-payer supporters hope that the good things in the ACA (no exclusion for pre-existing conditions, for example) will bring us closer to truly universal coverage. There are Twitter folk who think Obamacare could just be tweaked to offer plans that would bypass the private insurers–plans such as a Medicare buy-in or a public option. The physicians at point out the flaws in such proposals. See the 06/15/2010 article by Margaret Flowers, M.D.. During the debate on health care reform, Dr. Flowers was one of the “Baucus 8,” a respected physician removed from the Senate chamber when she and seven others asked to be allowed to testify on the merits of a single payer system. She explains why a publicly funded, nonprofit single payer cannot co-exist within a national system that includes the private insurers. Here’s the link to the article.