In the popular press, this is being (more-or-less accurately) portrayed as groups like Mayo Clinic, Cleveland Clinic, and Geisinger saying that they would not be able to participate in the proposed plans:
"Mayo Clinic, other premier medical groups call Obama plan on health care quality unworkable", Washington Post (AP), 5/12/2011.
The current administration has repeatedly tried to sell ACOs as a way to replicate the Mayo Clinic-type models nationally. But if even the Mayo Clinic states that they could not work under the proposed ACO guidelines, this could cast significant doubt about the idea.
AMGA online statement to members, 5/11/2011:
"AMGA Expresses Concerns about Accountable Care Organization Proposals to Donald Berwick, MD, CMS Administrator"
AMGA's letter letter to Medicare administrator Berwick (PDF)