I used to work at a porn company, so I'm not saying this out of any kind of prudishness, but I think porn on work computers is probably pretty bad (unless it's part of your job or you work from home, I mean).
I don't mean clicking on those distractedbyboobs links your friends send you, though. To me, those are are ok, as long as you're not being an idiot and offending any co-workers.
From my time looking at porn site analytics, though, it seems like there are a surprising number of people who spend the majority of their work day looking at porn. In fact, Monday was our heaviest sales day (weekends are the slowest sales days).
I mean, this isn't even taking into consideration people who look at child porn at work (which I unfortunately had to deal with at a different company).