I don't know about the N900 per se, but my PC's Audigy 2 card's breakout box lets out a "blast" of DC(resulting in a "click" noise) on the headphone port whenever it's powered on, or whenever the headphone cable is plugged in. Often it can be quite loud, too, depending on the hardware volume control POT.
Despite this, I haven't had any trouble with my Sennheiser HD 280PRO's, and have been using them for many months.

Most all audio devices will cause some sort of click as they are plugged in; I'd just make sure that I could replace your headphones if they ever fail and ignore it - Remember, headphones can fail from being dropped too often, or having the cord die...