I don't really think it's possible to be "raped" over a virtual enviroment unless actively 'role playing' it. Like EB said we all have the option to turn off the computer, close the IM or block the person from contacting us. Even if it's a child or a mentally ill person, I still don't consider it rape. If a child sees something they shouldn't see or speaks to someone they shouldn't, the parent isn't watching that child close enough, or the computer isn't set up to protect from those sorts of things. I know this from experience. I used to be a 12 year old looking around on porn websites behind my parents backs because I was curious. I never got caught, either. All you had to do was lie and say you were 18, it's easy enough.

Never having been raped, the only way I see rape is if it is physical with a mental component involved, and sometimes just physical. Frankly, online enviroments are mental. Someone can't reach through the computer and grab you; they can tell you that they're doing that, but there is no physical contact.