I have always ignored the porn that inevitably sneaks its way into newsgroups, spam-mail, etc. I have lived by the rule that if I don't like it, I do not have to download it.
I approached my use of Gnutella/Limewire in the same fashion. When files come up on a search that are of an adult orientation, ignore such and download what you are looking for (or refine your searches).
However one of the coolest things about gnutella (passing searches directly onto your peers/acting as a router for said searches), also makes it the thing which has made me decide to kill it from my system.
Monitor your searches, and you will see that you are facilitating the distribution of child porn. It doesn't matter that you do not have such on your machine. The client running on your machine is happily passing on the nastiest search terms possible to your peers...and you have no say in it. The manner of the architecture not only allows you to become an unwitting participant in the distribution of child pornography, but *forces* you to do such as you cannot set filters on the searches you are passing on to others.
If some "dirty old man" came to your door and asked you if you had any pictures or movies of naked children, your first response would be to call the authorities.
For those who still wish to utilize gnutella without the vendors doing the responsible thing and making it so you are not an unwitting participant in child porn, do what I did and start downloading a few of these things so you can get a list of IP address and then go to www.missingkids.com and report the scumbags trading this crap around. Even better...why don't the vendors or some other hardy soul develop or modify a client which will capturing the IPs of those searching for this crap and automatically forward on to law enforcement.