This just in:
RAPE STILL ISN'T FUNNY!
The truth of the matter is, in improv, technically nothing should be off-limits. So it's a sticky situation. If you can handle it with grace and sensitivity, great. If not, why do you want to risk putting off your audience?
A well-considered blog posting by Jason Chin, a teacher at iO, on the topic of rape and other hot-button issues played in improv, can be read here.