Several Japanese seiyuu are also singers, performing the OP/ED themes to the anime titles they are working (HAYASHIBARA Megumi springs to mind, but I could list quite a few).
Now, in the U.S., there is a larger VA market than just anime or cartoons. I didn't know this, but there's a lot more voice over work done in movies and shows than you might think. Anne Lockhart (Sheba from the original Battlestar Galactica) was at Starfest 2008 and told us that she's done all sorts of things like the Predator noises in Predator 2, the chimps in Project X and crowd voices in all kinRAB of other movies (something I had no idea ever happened). There's enough of that kind of work to keep her busy and pay the bills apparently.