I am currently enrolled in Medicare Part A, Part B, and Part D - but not in a Medicare Advantage plan. Will I be required to enroll in some private HMO or PPO when the ObamaCare plan is put in full action?
No. There is nothing in the Affordable Care Act requiring you to join a Medicare Advantage plan (private HMO or PPO). You will be able to continue with Original Medicare Part A, Part B and join a stand-alone Medicare Part D prescription drug plan - just as you have now.
As noted on the Medicare.gov site under the "Top 5 things to know about the Affordable Care Act (ACA) if you have Medicare:
Your Medicare coverage is protected.
Medicare isn’t part of the Health Insurance Marketplace established by ACA, so you don't have to replace your Medicare coverage with Marketplace coverage. No matter how you get Medicare, whether through Original Medicare or a Medicare Advantage Plan, you’ll still have the same benefits and security you have now.
You don’t need to do anything with the Marketplace during Open Enrollment. ..."