Plutonium 238 is used to make radiothermal generators for spacecraft. The decay of the Pu-238 atoms generates heat, and the temperature differential between the hot core and the infinite blackness of outer space! is sufficient to allow the generation of electricity.
But the Department of Defense has been out of the Pu-238 business since 1988, and we've been buying the stuff from Russia since 1992 to supplement the dwindling domestic supply of the stuff.
NASA Administrator Mike Griffin apparently doesn't like the idea of the US space program having to rely on foreign countries for space resources, such as flights and materials. Particularly he seems bothered by having to go to Russia for these things. I don't blame him; America is rich enough and has a long enough track record in space that in the year two-thousand-fricking-eight AD we ought to be able to build Hummer H2s on the Moon and ship them to Earth and still make a profit on the deal. WTF.
Meanwhile, the Russians have been using basically the same designs since Gagarin was being fitted for his "Hero of the Soviet Union" medal; despite an economy which totters on the brink of being third world Russia nonetheless manages, somehow, to keep flying rockets and--in a development which would have surprised the greats of the US space program in the 1960s--ends up being the go-to guy for space flights. Again, WTF.
This is what happens when you let government design a replacement for a perfectly viable launch platform by committee.