One thing that really interests me about this overall topic is the confluence of the casual usage of "rape" with other terms like "gay" or "retarded." In the case of the latter two, you have people using terms referring to groups of people in a pejorative sense, usually meaning "stupid." At least in my view, and I'd think in most other people's, the main point of offense there is that you're associating a group term that should be neutral at worst with something negative...in other words, by doing something like associating "gay" with "stupid," you're (perhaps unintentionally) implying that homosexuals as a whole are also stupid. Obviously, that's a bad thing.
When it comes to a term like "rape," though, the situation is subtly different. There, you're using a term that defines a horrifically negative action to describe something else negative...obviously not nearly to the same degree, but in a very exaggerated sense. In this case, the offense doesn't arise from associating a neutral term with a negative target (since I think any civilized, informed individual would agree that rape is horrible), but with the term itself. I'm not saying that casual usage of "rape" is any more or less appropriate than those other terms, or that the concerns raised in this thread aren't completely valid, but I think there is a difference at work here, though perhaps an inconsequential one.
