This Kaiser Family Foundation report outlines the gaps in coverage between Medicare Prescription Drug Benefit Program (Part D) and Medicare Advantage Prescription Drug plans available in 2010.

From the report: 

Gap coverage, when offered, is typically quite limited. In 2010, as in previous years, the vast majority of both PDPs and MA-PD plans that offer gap coverage cover generic drugs but not brand-name drugs. Among those PDPs offering gap coverage of generic drugs, the share covering all generics in the gap has declined from 14 percent in 2008 to 2 percent in 2010; similarly, the share of MA-PD plans covering all generics in the gap has dropped from 25 percent to 7 percent. Furthermore, plans offering gap coverage for generic drugs have covered a smaller share of generics over time.

Full report: Medicare Part D Data Spotlight: The Coverage Gap (PDF | 80.87 KB)

The Kaiser Family Foundation. (2009). Medicare Part D: the coverage gap. Hoadley, J., Summer, L., Hargrave, E., Cubanski, J. & Neuman, T.