AARP's endorsement indicates that it thinks one of two things will occur: (1) the proposed medicare savings (which represent the majority of the cost savings in the health care reform bill) will not affect seniors care; or, (2) the congress will never pursue the cost savings proposed (a la the annual kabuki with provider reimbursement rate cuts). This may be cynical, but I think AARP envisions the latter not the former transpiring.
Health Care reform would represent a boon to doctors in that it would expand the market for their services. There would be 21 million new, paying customers which would increase their overall volume and also the prices they can command. In theory, this would be mitigated to some degree by Medicare reimbursement cuts and reforms; but I suspect, the AMA also recognizes that these aspects of the health care reform are unlikely to materialize.