at the data center where I used to work, the FBI had taps on a couple servers and would pop in a couple times a year to confiscate client hard drives... wouldn't surprise me if child porn was involved (though all that was above my paygrade... my instructions were to just give them whatever they needed).
I had a roommate in college who used my computer to look at kiddie porn. I never did anything about it because it wasn't anything actually illegal, just erotica and "suggestive" but fully-clothed pictures, though I did password protect my box and asked him not to use it.
if it was something illegal, I think I'd escalate it to the appropriate authorities (ie: my boss if it were a client, the fuzz if it was something outside of work)