I use Peltor noise cancellation headphones to absorb air conditioner and vehicle noise (coming from outside the building, I don't use them while driving). I find they are really terrible about picking up 60 cycle a/c line hum from my Lenovo laptop and Asus EEE netbook headphone out, though surprisingly not from my officemate's Mac pro. I bought a Headroom Total Bithead dac/amp with the idea of pure digital coming from the computer, but found that the 60 cycle crap still comes through the dac/amp, a sign of cheesy design. Maybe the Pico is better. I don't notice this effect with passive headphones but in the environments I listen in, the ANC is indispensable no matter how badly it otherwise messes up the SQ.
I also found that there were occasionally audio glitches, especially if I did something demanding on the computer. I could use a separate computer just for audio, but in the end I said "screw it" abd bought a Cowon D2, which is what I use now and am pretty happy with.