I can't find a reference to it now, but there was talk some months back about splitting the space nuclear power and electric propulsion portions of Prometheus...it looks like it may indeed happen under the coming 2006 budget: NASA 2006 Budget Presented: Hubble, Nuclear Initiative Suffer

The biggest casualty of NASA's latest budget request is the Jupiter Icy Moons Orbiter (JIMO) mission, which is effectively canceled in the 2006 request. NASA had planned to launch the multi-billion dollar probe around 2015 as the Project Prometheus nuclear power and propulsion initiative's flagship demonstration.

NASA officials now say JIMO is too ambitious an undertaking for an initial demonstration, and a search for an alternative mission is underway. NASA is still requesting $320 million to continue nuclear power and propulsion work in 2006, but that is substantially lower than the roughly $500 million the agency had planned to spend based on last year's five-year budget projection.