The heroine in Advanced Physical Chemistry doesn't swear, and I'm pretty sure this is the most I've ever had to stretch myself as a writer. I've gotten into the heads of superheroes, time travelers, geniuses, criminals, and historical figures in my writing with no problem. But a character who doesn't curse? That's the one who's turning out to be hard for me to relate to.
It is pretty fun coming up with non-swear alternatives, though. I may adopt some of them in real life and clean up my potty mouth a little.