I just cut myself on some rusty metal, should I be ok? How rare is tetanus in the u.S.?

Rare because of shot. Tetanus spores are everywhere in nature and in high levels in some sites like the soil. Any break in the skin can allow tetanus spores in, where they create an infection that produces tetanus toxin/tetanus as a disease. As long as you completed a primary series & booster within 10 yrs you should be fine. If not get one within a few days. Tend the wound & observe for any sign of infection.