Yeah, porn can definitely be an addiction. Some people can just look at a Playboy or Penthouse once in a while, no problem. For others it is a serious issue. For them, actual intimacy can be very difficult and just looking at the computer or DVDs is easier than actually "performing". I think in addition to an addiction that affects the pleasure centers of the brain similar to drugs, it is also a form of anti-social behaviour. Much like addiction to violent video games.
And of course, not that this may be of much importance at this stage, but looking at Internet porn sites can really screw up your computer with the presence of spyware, adware, trojans, and worms.