Russia and the United States are discussing a nuclear cooperation agreement that will provide the necessary legal framework for any future cooperation in the area of nuclear power. The United States has traditionally considered the agreement as an instrument that would help it persuade Russia to curtail its nuclear work in Iran, arguing that the United States could potentially provide Russia with a much broader range of business opportunities than Iran. That did not impress Russia, which probably sensed that the politics of nuclear cooperation with the United States would be extremely difficult anyway, with or without the agreement.
At this point, however, Iran is hardly the reason why the United States and Russia are talking seriously about the agreement - the situation there is long past the point where Russia's work in Bushehr could make any difference. It is much more likely that the United States needs the agreement to move ahead with its Global Nuclear Energy Partnership program. In particular, the cooperation agreement will allow a whole range of spent fuel storage and disposal arrangements that would include interim storage of the U.S.-consent-rights fuel on Russian territory. This may seem unlikely at the moment, but let's don't forget that about six month ago a serious discussion of the nuclear cooperation agreement seemed quite unlikely as well.