Generally, dust is pretty innocuous, being relatively non-condutive. Try looking inside the back of a tube-TV after a couple of years' use.
Of course a TV tube has a rather high voltage - 25kV - which attracts dust electrostatically (and could be a fire hazard). The Pi doesn't have that so won't suffer so much.
Also, there's a lot less power around, and it doesn't get hot, so the fire hazard is so low as to be nonexistant.
One problem with dust is with mechanical parts - connectors particularly don't like it. But unless you eat breakfast over it, oxidation of the surfaces is going to be a bigger problem.
In your desktop PC or your laptop, the major culprit for dust-gathering is the fan. It continually provides fresh dust-laden air, with heatsinks especially designed to break up the air flow so the dust can settle in the fingers. Since the dust has low thermal conductivity, the chips get hotter, so the fan blows harder, increasing the dust-gathering
You can increase your dust-gathering by having a barbecue - the volatile hydrocarbons will condense on the dust and cure into a really lovely gunge.
Unfortunately, the Pi doesn't have a fan, nor does it have any heatsink, so you won't see the full glory of gunge on it unless you live in Glasgow and deep-fry your Pi.