If you decide to use the Aetna Integration Plan as you supplemental medical coverage then you will need to buy a Part D prescription drug plan from a private insurer. You HAVE to buy a drug plan to maintain continuous coverage. If you don't have a drug plan there will be a penalty applied of 1%/month for every month you are not covered by a part D plan if you change your mind. It is not a huge penalty because the monthly premiums are low and it is based on the national average which is about $33/month ... but it will cost about $4 per year for every year you are not covered and can add up. The penalty doesn't goes away (unless you fall below the poverty line and qualify for low income programs.) Some people who don't really want a drug plan buy the cheapest plan they can find in their zip code to avoid the lifetime penalty. Some premiums are as low as $15/month.
In 2011 there are changes to Part D that can lower your out of pocket costs if you do get into the doughnut hole. The old rule was that once you were in the doughnut hole you paid the entire bill for your drugs until you hit an out of pocket limit (about $4000). Granted you pay the insurer's discount rate for the drug - but the insurance companies have not been too good at negotiating discounts. Starting in 2011you will pay 50% of the insurer discount rate and also "get credit" for 100% of the cost toward getting out of the doughnut hole. For generics you only get a 7% break - I will guess because generics are much cheaper.
All of these pricing features are in the "configurator" planfinder on medicare.gov. Use it to determine your total cost for a given drug plan.