Spaceflight Now has an article that provides an update on the plutonium-238 supply problems. This radioisotope is used as a power source for missions where solar power is impracticle or impossible. Currently, the US does not have enough Pu-238 to enable the proposed Jupiter Europa Orbiter (JEO) mission that NASA would like to fly at the end of the decade. According to the article, if Russia either agrees to resume selling Pu-238 to the US, or Congress approves start up of new Pu-238 production in the FY11 budget, then sufficient Pu-238 supplies will be on hand for the JEO mission. [Editorial thought: While NASA would probably prefer to have both sources of Pu-238, restarting production probably would be the more desirable alternative of the two. Russia no longer produces Pu-238, so NASA would like to have an ongoing source of new material.]
In the mean time, the European Space Agency would like to begin flying missions that require radioisotope power in the 2020s. It apparently has concluded that it should not depend on the US supplying Pu-238. (Besides the uncertainties of US budgets, my understanding of US law is that the US cannot provide Pu-238 for launch on a foreign launch vehicle. ESA, I presume, would like to use its own launchers.) ESA is looking at an alternative radioisotope, americium-241. This radioisotope has a longer half life than Pu-238, but produces less heat and therefore fewer watts of electricity. It appears that ESA is currently in the early exploratory phase of this idea, and it is not yet a committed, funded project.
You can read the entire article at http://spaceflightnow.com/news/n1007/09rtg/