Just as an FYI, Sheril at the Intersection has created this extremely helpful list of policy fellowships for scientists and engineers. It's definitely worth bookmarking if you have any interest in exploring science policy. The fellowships on this list range from weeks to years, and placements are available throughout the federal government, in science agencies and on the Hill.
This chart shows where your federal tax dollars go, based on Feb 2008 budget numbers (note: this shows discretionary budget, or appropriations, only - go here for more details, or look at the inset in the lower right corner to see where the other two-thirds of the budget are). Visit wallstats.com for a giant version.