I sent a couple and every time I get "Your Abuse Report has been investigated and resolved". While I understand that you don't necessarily want to encourage people running around and ARing anything that might be offensive, this isn't very good either, as it's very discouraging. Some jerk threw me two sims away while I was quietly scripting in my home, and then all I get is the confirmation email and nothing at all in the police blotter. Makes me feel ignored.
I would like at the very least an indication of whether the AR was acted on in any way, or ignored completely. If ignored, it'd be nice to know why: not abuse, not enough data, you AR too much so we just decided to ignore you completely, etc.
If you really really don't want to provide feedback on specific ARs, how about aggregate statistics? Say, every week update a counter accessible from the website with the number of ARs, how many have been acted on, and how many were ignored. This way at least it'd be possible to tell whether it's worth bothering with them or not.